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Reflect in the virtiofs documentation that virtiofs can now be used even without NUMA. While at it, be more precise where and why shared memory is required. Signed-off-by: Halil Pasic <pasic@linux.ibm.com> Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com> Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
151 lines
3.8 KiB
ReStructuredText
151 lines
3.8 KiB
ReStructuredText
============================
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Sharing files with Virtio-FS
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============================
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.. contents::
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Virtio-FS
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=========
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Virtio-FS is a shared file system that lets virtual machines access
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a directory tree on the host. Unlike existing approaches, it
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is designed to offer local file system semantics and performance.
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See https://virtio-fs.gitlab.io/
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Host setup
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==========
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Almost all virtio devices (all that use virtqueues) require access to
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at least certain portions of guest RAM (possibly policed by DMA). In
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case of virtiofsd, much like in case of other vhost-user (see
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https://www.qemu.org/docs/master/interop/vhost-user.html) virtio
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devices that are realized by an userspace process, this in practice
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means that QEMU needs to allocate the backing memory for all the guest
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RAM as shared memory. As of QEMU 4.2, it is possible to explicitly
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specify a memory backend when specifying the NUMA topology. This
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method is however only viable for machine types that do support
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NUMA. As of QEMU 5.0.0, it is possible to specify the memory backend
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without NUMA (using the so called memobject interface).
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Either of the following:
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* Use file-backed memory
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Configure the directory where the files backing the memory will be stored
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with the ``memory_backing_dir`` option in ``/etc/libvirt/qemu.conf``
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::
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# This directory is used for memoryBacking source if configured as file.
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# NOTE: big files will be stored here
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memory_backing_dir = "/dev/shm/"
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* Use hugepage-backed memory
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Make sure there are enough huge pages allocated for the requested guest memory.
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For example, for one guest with 2 GiB of RAM backed by 2 MiB hugepages:
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::
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# virsh allocpages 2M 1024
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Guest setup
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===========
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#. Specify the NUMA topology (this step is only required for the NUMA case)
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in the domain XML of the guest.
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For the simplest one-node topology for a guest with 2GiB of RAM and 8 vCPUs:
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::
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<domain>
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...
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<cpu ...>
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<numa>
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<cell id='0' cpus='0-7' memory='2' unit='GiB' memAccess='shared'/>
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</numa>
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</cpu>
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...
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</domain>
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Note that the CPU element might already be specified and only one is allowed.
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#. Specify the memory backend
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Either of the following:
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* File-backed memory
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::
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<domain>
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...
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<memoryBacking>
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<access mode='shared'/>
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</memoryBacking>
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...
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</domain>
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This will create a file in the directory specified in ``qemu.conf``
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* Hugepage-backed memory
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::
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<domain>
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...
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<memoryBacking>
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<hugepages>
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<page size='2' unit='M'/>
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</hugepages>
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<access mode='shared'/>
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</memoryBacking>
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...
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</domain>
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#. Add the ``vhost-user-fs`` QEMU device via the ``filesystem`` element
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::
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<domain>
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...
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<devices>
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...
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<filesystem type='mount' accessmode='passthrough'>
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<driver type='virtiofs'/>
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<source dir='/path'/>
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<target dir='mount_tag'/>
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</filesystem>
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...
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</devices>
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</domain>
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Note that despite its name, the ``target dir`` is actually a mount tag and does
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not have to correspond to the desired mount point in the guest.
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So far, ``passthrough`` is the only supported access mode and it requires
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running the ``virtiofsd`` daemon as root.
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#. Boot the guest and mount the filesystem
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::
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guest# mount -t virtiofs mount_tag /mnt/mount/path
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Note: this requires virtiofs support in the guest kernel (Linux v5.4 or later)
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Optional parameters
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===================
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More optional elements can be specified
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::
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<driver type='virtiofs' queue='1024'/>
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<binary path='/usr/libexec/virtiofsd' xattr='on'>
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<cache mode='always'/>
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<lock posix='on' flock='on'/>
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</binary>
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