mirror of
https://gitlab.com/libvirt/libvirt.git
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8e02fa2e25
Commit 50dfabbb59 forgot to add this important bit on how to check that
all the changes to the XML actually worked.
Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
534 lines
18 KiB
XML
534 lines
18 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html>
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<html xmlns="http://www.w3.org/1999/xhtml">
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<body>
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<h1>Launch security with AMD SEV</h1>
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<ul id="toc"></ul>
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<p>
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Storage encryption in modern public cloud computing is a common practice.
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However, from the point of view of a user of these cloud workloads, a
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significant amount of trust needs to be put in the cloud platform security as
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well as integrity (was the hypervisor tampered?). For this reason there's ever
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rising demand for securing data in use, i.e. memory encryption.
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One of the solutions addressing this matter is AMD SEV.
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</p>
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<h2>AMD SEV</h2>
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<p>
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SEV (Secure Encrypted Virtualization) is a feature extension of AMD's SME (Secure
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Memory Encryption) intended for KVM virtual machines which is supported
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primarily on AMD's EPYC CPU line. In contrast to SME, SEV uses a unique memory encryption
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key for each VM. The whole encryption of memory pages is completely transparent
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to the hypervisor and happens inside dedicated hardware in the on-die memory controller.
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Each controller includes a high-performance Advanced Encryption Standard
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(AES) engine that encrypts data when it is written to DRAM and decrypts it
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when read.
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For more details about the technology itself, you can visit
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<a href="https://developer.amd.com/sev/">AMD's developer portal</a>.
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</p>
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<h2><a id="Host">Enabling SEV on the host</a></h2>
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<p>
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Before VMs can make use of the SEV feature you need to make sure your
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AMD CPU does support SEV. You can check whether SEV is among the CPU
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flags with:
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</p>
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<pre>
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$ cat /proc/cpuinfo | grep sev
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...
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sme ssbd sev ibpb</pre>
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<p>
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Next step is to enable SEV in the kernel, because it is disabled by default.
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This is done by putting the following onto the kernel command line:
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</p>
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<pre>
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mem_encrypt=on kvm_amd.sev=1
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</pre>
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<p>
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To make the changes persistent, append the above to the variable holding
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parameters of the kernel command line in
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<code>/etc/default/grub</code> to preserve SEV settings across reboots
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</p>
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<pre>
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$ cat /etc/default/grub
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...
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GRUB_CMDLINE_LINUX="... mem_encrypt=on kvm_amd.sev=1"
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$ grub2-mkconfig -o /boot/efi/EFI/<distro>/grub.cfg</pre>
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<p>
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<code>mem_encrypt=on</code> turns on the SME memory encryption feature on
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the host which protects against the physical attack on the hypervisor
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memory. The <code>kvm_amd.sev</code> parameter actually enables SEV in
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the kvm module. It can be set on the command line alongside
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<code>mem_encrypt</code> like shown above, or it can be put into a
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module config under <code>/etc/modprobe.d/</code>
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</p>
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<pre>
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$ cat /etc/modprobe.d/sev.conf
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options kvm_amd sev=1
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</pre>
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<p>
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After rebooting the host, you should see SEV being enabled in the kernel:
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</p>
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<pre>
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$ cat /sys/module/kvm_amd/parameters/sev
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1
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</pre>
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<h2><a id="Virt">Checking SEV support in the virt stack</a></h2>
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<p>
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<b>Note: All of the commands bellow need to be run with root privileges.</b>
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</p>
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<p>
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First make sure you have the following packages in the specified versions:
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</p>
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<ul>
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<li>
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libvirt >= 4.5.0 (>5.1.0 recommended due to additional SEV bugfixes)
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</li>
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<li>
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QEMU >= 2.12.0
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</li>
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</ul>
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<p>
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To confirm that the virtualization stack supports SEV, run the following:
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</p>
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<pre>
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# virsh domcapabilities
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<domainCapabilities>
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...
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<features>
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...
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<sev supported='yes'>
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<cbitpos>47</cbitpos>
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<reducedPhysBits>1</reducedPhysBits>
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</sev>
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...
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</features>
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</domainCapabilities></pre>
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<p>
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Note that if libvirt was already installed and libvirtd running before enabling SEV in the kernel followed by the host reboot you need to force libvirtd
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to re-probe both the host and QEMU capabilities. First stop libvirtd:
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</p>
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<pre>
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# systemctl stop libvirtd.service
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</pre>
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<p>
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Now you need to clean the capabilities cache:
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</p>
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<pre>
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# rm -f /var/cache/libvirt/qemu/capabilities/*
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</pre>
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<p>
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If you now restart libvirtd, it will re-probe the capabilities and if
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you now run:
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</p>
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<pre>
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# virsh domcapabilities
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</pre>
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<p>
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SEV should be listed as supported. If you still see:
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</p>
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<pre>
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<sev supported='no'/>
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</pre>
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<p>
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it means one of two things:
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<ol>
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<li>
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libvirt does support SEV, but either QEMU or the host does not
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</li>
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<li>
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you have libvirt <=5.1.0 which suffered from getting a
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<code>'Permission denied'</code> on <code>/dev/sev</code> because
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of the default permissions on the character device which prevented
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QEMU from opening it during capabilities probing - you can either
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manually tweak the permissions so that QEMU has access to it or
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preferably install libvirt 5.1.0 or higher
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</li>
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</ol>
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</p>
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<h2><a id="Configuration">VM Configuration</a></h2>
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<p>
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SEV is enabled in the XML by specifying the
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<a href="https://libvirt.org/formatdomain.html#launchSecurity"><launchSecurity> </a> element. However, specifying <code>launchSecurity</code> isn't
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enough to boot an SEV VM. Further configuration requirements are discussed
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below.
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</p>
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<h3><a id="Machine">Machine type</a></h3>
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<p>
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Even though both Q35 and legacy PC machine types (for PC see also
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"virtio") can be used with SEV, usage of the legacy PC machine type is
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strongly discouraged, since depending on how your OVMF package was
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built (e.g. including features like SecureBoot or SMM) Q35 may even be
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required.
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</p>
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<h5>Q35</h5>
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<pre>
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...
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<os>
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<type arch='x86_64' machine='pc-q35-3.0'>hvm</type>
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...
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</os>
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...</pre>
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<h5>i440fx (discouraged)</h5>
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<pre>
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...
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<os>
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<type arch='x86_64' machine='pc-i440fx-3.0'>hvm</type>
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...
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</os>
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...
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</pre>
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<h3><a id="Boot">Boot loader</a></h3>
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<p>
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SEV is only going to work with OVMF (UEFI), so you'll need to point libvirt to
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the correct OVMF binary.
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</p>
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<pre>
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...
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<os>
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<type arch='x86_64' machine='pc-q35-3.0'>hvm</type>
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<loader readonly='yes' type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
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</os>
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...</pre>
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<h3><a id="Memory">Memory</a></h3>
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<p>
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Internally, SEV expects that the encrypted memory pages won't be swapped out or move
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around so the VM memory needs to be pinned in physical RAM which will be
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handled by QEMU. Apart from that, certain memory regions allocated by QEMU
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itself (UEFI pflash, device ROMs, video RAM, etc.) have to be encrypted as
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well. This causes a conflict in how libvirt tries to protect the host.
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By default, libvirt enforces a memory hard limit on each VM's cgroup in order
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to protect the host from malicious QEMU to allocate and lock all the available
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memory. This limit corresponds to the total memory allocation for the VM given
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by <code><currentMemory></code> element. However, trying to account for the additional
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memory regions QEMU allocates when calculating the limit in an automated manner
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is non-deterministic. One way to resolve this is to set the hard limit manually.
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<p>
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Note: Figuring out the right number so that your guest boots and isn't killed is
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challenging, but 256MiB extra memory over the total guest RAM should suffice for
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most workloads and may serve as a good starting point.
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For example, a domain with 4GB memory with a 256MiB extra hard limit would look
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like this:
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</p>
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</p>
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<pre>
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# virsh edit <domain>
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<domain>
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...
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<currentMemory unit='KiB'>4194304</currentMemory>
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<memtune>
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<hard_limit unit='KiB'>4456448</hard_limit>
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</memtune>
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...
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</domain></pre>
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<p>
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There's another, preferred method of taking care of the limits by
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using the<code><memoryBacking></code> element along with the
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<code><locked/></code> subelement:
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</p>
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<pre>
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<domain>
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...
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<memoryBacking>
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<locked/>
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</memoryBacking>
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...
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</domain></pre>
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<p>
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What that does is that it tells libvirt not to force any hard limit (well,
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unlimited) upon the VM cgroup. The obvious advantage is that one doesn't need
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to determine the hard limit for every single SEV-enabled VM. However, there is
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a significant security-related drawback to this approach. Since no hard limit
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is applied, a malicious QEMU could perform a DoS attack by locking all of the
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host's available memory. The way to avoid this issue and to protect the host is
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to enforce a bigger hard limit on the master cgroup containing all of the VMs
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- on systemd this is <code>machine.slice</code>.
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</p>
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<pre>
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# systemctl set-property machine.slice MemoryHigh=<value></pre>
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<p>
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To put even stricter measures in place which would involve the OOM killer, use
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<pre>
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# systemctl set-property machine.slice MemoryMax=<value></pre>
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instead. Alternatively, you can create a systemd config (don't forget
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to reload systemd configuration in this case):
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<pre>
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# cat << EOF > /etc/systemd/system.control/machine.slice.d/90-MemoryMax.conf
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MemoryMax=<value>
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EOF</pre>
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The trade-off to keep in mind with the second approach is that the VMs
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can still perform DoS on each other.
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</p>
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<h3><a id="Virtio">Virtio</a></h3>
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<p>
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In order to make virtio devices work, we need to enable emulated IOMMU
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on the devices so that virtual DMA can work.
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</p>
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<pre>
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# virsh edit <domain>
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<domain>
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...
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<controller type='virtio-serial' index='0'>
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<driver iommu='on'/>
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</controller>
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<controller type='scsi' index='0' model='virtio-scsi'>
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<driver iommu='on'/>
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</controller>
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...
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<memballoon model='virtio'>
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<driver iommu='on'/>
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</memballoon>
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<rng model='virtio'>
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<backend model='random'>/dev/urandom</backend>
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<driver iommu='on'/>
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</rng>
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...
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<domain></pre>
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<p>
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If you for some reason want to use the legacy PC machine type, further changes
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to the virtio
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configuration is required, because SEV will not work with Virtio <1.0. In
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libvirt, this is handled by using the virtio-non-transitional device model
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(libvirt >= 5.2.0 required).
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<p>
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Note: some devices like video devices don't
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support non-transitional model, which means that virtio GPU cannot be used.
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</p>
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</p>
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<pre>
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<domain>
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...
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<devices>
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...
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<memballoon model='virtio-non-transitional'>
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<driver iommu='on'/>
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</memballoon>
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</devices>
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...
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</domain></pre>
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<h2><a id="Guest">Checking SEV from within the guest</a></h2>
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<p>
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After making the necessary adjustments discussed in
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<a href="#Configuration">Configuration</a>, the VM should now boot
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successfully with SEV enabled. You can then verify that the guest has
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SEV enabled by running:
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</p>
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<pre>
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# dmesg | grep -i sev
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AMD Secure Encrypted Virtualization (SEV) active</pre>
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<h2><a id="Limitations">Limitations</a></h2>
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<p>
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Currently, the boot disk cannot be of type virtio-blk, instead, virtio-scsi
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needs to be used if virtio is desired. This limitation is expected to be lifted
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with future releases of kernel (the kernel used at the time of writing the
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article is 5.0.14).
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If you still cannot start an SEV VM, it could be because of wrong SELinux label on the <code>/dev/sev</code> device with selinux-policy <3.14.2.40 which prevents QEMU from touching the device. This can be resolved by upgrading the package, tuning the selinux policy rules manually to allow svirt_t to access the device (see <code>audit2allow</code> on how to do that) or putting SELinux into permissive mode (discouraged).
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</p>
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<h2><a id="Examples">Full domain XML examples</a></h2>
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<h5>Q35 machine</h5>
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<pre>
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<domain type='kvm'>
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<name>sev-dummy</name>
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<memory unit='KiB'>4194304</memory>
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<currentMemory unit='KiB'>4194304</currentMemory>
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<memoryBacking>
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<locked/>
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</memoryBacking>
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<vcpu placement='static'>4</vcpu>
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<os>
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<type arch='x86_64' machine='pc-q35-3.0'>hvm</type>
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<loader readonly='yes' type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
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<nvram>/var/lib/libvirt/qemu/nvram/sev-dummy_VARS.fd</nvram>
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</os>
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<features>
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<acpi/>
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<apic/>
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<vmport state='off'/>
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</features>
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<cpu mode='host-model' check='partial'>
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<model fallback='allow'/>
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</cpu>
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<clock offset='utc'>
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<timer name='rtc' tickpolicy='catchup'/>
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<timer name='pit' tickpolicy='delay'/>
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<timer name='hpet' present='no'/>
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</clock>
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<on_poweroff>destroy</on_poweroff>
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<on_reboot>restart</on_reboot>
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<on_crash>destroy</on_crash>
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<pm>
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<suspend-to-mem enabled='no'/>
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<suspend-to-disk enabled='no'/>
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</pm>
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<devices>
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<emulator>/usr/bin/qemu-kvm</emulator>
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<disk type='file' device='disk'>
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<driver name='qemu' type='qcow2'/>
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<source file='/var/lib/libvirt/images/sev-dummy.qcow2'/>
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<target dev='sda' bus='scsi'/>
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<boot order='1'/>
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</disk>
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<controller type='virtio-serial' index='0'>
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<driver iommu='on'/>
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</controller>
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<controller type='scsi' index='0' model='virtio-scsi'>
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<driver iommu='on'/>
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</controller>
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<interface type='network'>
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<mac address='52:54:00:cc:56:90'/>
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<source network='default'/>
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<model type='virtio'/>
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<driver iommu='on'/>
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</interface>
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<graphics type='spice' autoport='yes'>
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<listen type='address'/>
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<gl enable='no'/>
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</graphics>
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<video>
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<model type='qxl'/>
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</video>
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<memballoon model='virtio'>
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<driver iommu='on'/>
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</memballoon>
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<rng model='virtio'>
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<driver iommu='on'/>
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</rng>
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</devices>
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<launchSecurity type='sev'>
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<cbitpos>47</cbitpos>
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<reducedPhysBits>1</reducedPhysBits>
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<policy>0x0003</policy>
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</launchSecurity>
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</domain></pre>
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<h5>PC-i440fx machine:</h5>
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<pre>
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<domain type='kvm'>
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<name>sev-dummy-legacy</name>
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<memory unit='KiB'>4194304</memory>
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<currentMemory unit='KiB'>4194304</currentMemory>
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<memtune>
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<hard_limit unit='KiB'>5242880</hard_limit>
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</memtune>
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<vcpu placement='static'>4</vcpu>
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<os>
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<type arch='x86_64' machine='pc-i440fx-3.0'>hvm</type>
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<loader readonly='yes' type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
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<nvram>/var/lib/libvirt/qemu/nvram/sev-dummy_VARS.fd</nvram>
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<boot dev='hd'/>
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</os>
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<features>
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<acpi/>
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<apic/>
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<vmport state='off'/>
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</features>
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<cpu mode='host-model' check='partial'>
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<model fallback='allow'/>
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</cpu>
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<clock offset='utc'>
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<timer name='rtc' tickpolicy='catchup'/>
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<timer name='pit' tickpolicy='delay'/>
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<timer name='hpet' present='no'/>
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</clock>
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<on_poweroff>destroy</on_poweroff>
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<on_reboot>restart</on_reboot>
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<on_crash>destroy</on_crash>
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<pm>
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<suspend-to-mem enabled='no'/>
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<suspend-to-disk enabled='no'/>
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</pm>
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<devices>
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<emulator>/usr/bin/qemu-kvm</emulator>
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<disk type='file' device='disk'>
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<driver name='qemu' type='qcow2'/>
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<source file='/var/lib/libvirt/images/sev-dummy-seabios.qcow2'/>
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<target dev='sda' bus='sata'/>
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</disk>
|
|
<interface type='network'>
|
|
<mac address='52:54:00:d8:96:c8'/>
|
|
<source network='default'/>
|
|
<model type='virtio-non-transitional'/>
|
|
</interface>
|
|
<serial type='pty'>
|
|
<target type='isa-serial' port='0'>
|
|
<model name='isa-serial'/>
|
|
</target>
|
|
</serial>
|
|
<console type='pty'>
|
|
<target type='serial' port='0'/>
|
|
</console>
|
|
<input type='tablet' bus='usb'>
|
|
<address type='usb' bus='0' port='1'/>
|
|
</input>
|
|
<input type='mouse' bus='ps2'/>
|
|
<input type='keyboard' bus='ps2'/>
|
|
<graphics type='spice' autoport='yes'>
|
|
<listen type='address'/>
|
|
<gl enable='no'/>
|
|
</graphics>
|
|
<video>
|
|
<model type='qxl' ram='65536' vram='65536' vgamem='16384' heads='1' primary='yes'/>
|
|
</video>
|
|
<memballoon model='virtio-non-transitional'>
|
|
<driver iommu='on'/>
|
|
</memballoon>
|
|
<rng model='virtio-non-transitional'>
|
|
<driver iommu='on'/>
|
|
</rng>
|
|
</devices>
|
|
<launchSecurity type='sev'>
|
|
<cbitpos>47</cbitpos>
|
|
<reducedPhysBits>1</reducedPhysBits>
|
|
<policy>0x0003</policy>
|
|
</launchSecurity>
|
|
</domain></pre>
|
|
</body>
|
|
</html>
|