mirror of
https://github.com/cloud-hypervisor/cloud-hypervisor.git
synced 2024-11-05 03:21:13 +00:00
5c7164e55f
This functionality is deprecated. See: #2231 Signed-off-by: Rob Bradford <robert.bradford@intel.com>
210 lines
8.4 KiB
Markdown
210 lines
8.4 KiB
Markdown
# Virtual IOMMU
|
|
|
|
## Rationales
|
|
|
|
Having the possibility to expose a virtual IOMMU to the guest can be
|
|
interesting to support specific use cases. That being said, it is always
|
|
important to keep in mind a virtual IOMMU can impact the performance of the
|
|
attached devices, which is the reason why one should be careful when enabling
|
|
this feature.
|
|
|
|
### Protect nested virtual machines
|
|
|
|
The first reason why one might want to expose a virtual IOMMU to the guest is
|
|
to increase the security regarding the memory accesses performed by the virtual
|
|
devices (VIRTIO devices), on behalf of the guest drivers.
|
|
|
|
With a virtual IOMMU, the VMM stands between the guest driver and its device
|
|
counterpart, validating and translating every address before to try accessing
|
|
the guest memory. This is standard interposition that is performed here by the
|
|
VMM.
|
|
|
|
The increased security does not apply for a simple case where we have one VM
|
|
per VMM. Because the guest cannot be trusted, as we always consider it could
|
|
be malicious and gain unauthorized privileges inside the VM, preventing some
|
|
devices from accessing the entire guest memory is pointless.
|
|
|
|
But let's take the interesting case of nested virtualization, and let's assume
|
|
we have a VMM running a first layer VM. This L1 guest is fully trusted as the
|
|
user intends to run multiple VMs from this L1. We can end up with multiple L2
|
|
VMs running on a single L1 VM. In this particular case, and without exposing a
|
|
virtual IOMMU to the L1 guest, it would be possible for any L2 guest to use the
|
|
device implementation from the host VMM to access the entire guest L1 memory.
|
|
The virtual IOMMU prevents from this kind of trouble as it will validate the
|
|
addresses the device is authorized to access.
|
|
|
|
### Achieve VFIO nested
|
|
|
|
Another reason for having a virtual IOMMU is to allow passing physical devices
|
|
from the host through multiple layers of virtualization. Let's take as example
|
|
a system with a physical IOMMU running a VM with a virtual IOMMU. The
|
|
implementation of the virtual IOMMU is responsible for updating the physical
|
|
DMA Remapping table (DMAR) everytime the DMA mapping changes. This must happen
|
|
through the VFIO framework on the host as this is the only userspace interface
|
|
to interact with a physical IOMMU.
|
|
|
|
Relying on this update mechanism, it is possible to attach physical devices to
|
|
the virtual IOMMU, which allows these devices to be passed from L1 to another
|
|
layer of virtualization.
|
|
|
|
## Why virtio-iommu?
|
|
|
|
The Cloud Hypervisor project decided to implement the brand new virtio-iommu
|
|
device in order to provide a virtual IOMMU to its users. The reason being the
|
|
simplicity brought by the paravirtualization solution. By having one side
|
|
handled from the guest itself, it removes the complexity of trapping memory
|
|
page accesses and shadowing them. This is why the project will not try to
|
|
implement a full emulation of a physical IOMMU.
|
|
|
|
## Pre-requisites
|
|
|
|
### Kernel
|
|
|
|
Since virtio-iommu has landed partially into the version 5.3 of the Linux
|
|
kernel, a special branch is needed to get things working with Cloud Hypervisor.
|
|
By partially, we are talking about x86 specifically, as it is already fully
|
|
functional for ARM architectures.
|
|
|
|
## Usage
|
|
|
|
In order to expose a virtual IOMMU to the guest, it is required to create a
|
|
virtio-iommu device and expose it through the ACPI IORT table. This can be
|
|
simply achieved by attaching at least one device to the virtual IOMMU.
|
|
|
|
The way to expose to the guest a specific device as sitting behind this IOMMU
|
|
is to explicitly tag it from the command line with the option `iommu=on`.
|
|
|
|
Not all devices support this extra option, and the default value will always
|
|
be `off` since we want to avoid the performance impact for most users who don't
|
|
need this.
|
|
|
|
Refer to the command line `--help` to find out which device support to be
|
|
attached to the virtual IOMMU.
|
|
|
|
Below is a simple example exposing the `virtio-blk` device as attached to the
|
|
virtual IOMMU:
|
|
|
|
```bash
|
|
./cloud-hypervisor \
|
|
--cpus boot=1 \
|
|
--memory size=512M \
|
|
--disk path=focal-server-cloudimg-amd64.raw,iommu=on \
|
|
--kernel custom-vmlinux \
|
|
--cmdline "console=ttyS0 console=hvc0 root=/dev/vda1 rw" \
|
|
```
|
|
|
|
From a guest perspective, it is easy to verify if the device is protected by
|
|
the virtual IOMMU. Check the directories listed under
|
|
`/sys/kernel/iommu_groups`:
|
|
|
|
```bash
|
|
ls /sys/kernel/iommu_groups
|
|
0
|
|
```
|
|
|
|
In this case, only one IOMMU group should be created. Under this group, it is
|
|
possible to find out the b/d/f of the device(s) part of this group.
|
|
|
|
```bash
|
|
ls /sys/kernel/iommu_groups/0/devices/
|
|
0000:00:03.0
|
|
```
|
|
|
|
And you can validate the device is the one we expect running `lspci`:
|
|
|
|
```bash
|
|
lspci
|
|
00:00.0 Host bridge: Intel Corporation Device 0d57
|
|
00:01.0 Unassigned class [ffff]: Red Hat, Inc. Device 1057
|
|
00:02.0 Unassigned class [ffff]: Red Hat, Inc. Virtio console
|
|
00:03.0 Mass storage controller: Red Hat, Inc. Virtio block device
|
|
00:04.0 Unassigned class [ffff]: Red Hat, Inc. Virtio RNG
|
|
```
|
|
|
|
## Faster mappings
|
|
|
|
By default, the guest memory is mapped with 4k pages and no huge pages, which
|
|
causes the virtual IOMMU device to be asked for 4k mappings only. This
|
|
configuration slows down the setup of the physical IOMMU as an important number
|
|
of requests need to be issued in order to create large mappings.
|
|
|
|
One use case is even more impacted by the slowdown, the nested VFIO case. When
|
|
passing a device through a L2 guest, the VFIO driver running in L1 will update
|
|
the DMAR entries for the specific device. Because VFIO pins the entire guest
|
|
memory, this means the entire mapping of the L2 guest need to be stored into
|
|
multiple 4k mappings. Obviously, the bigger the L2 guest RAM is, the longer the
|
|
update of the mappings will last. There is an additional problem happening in
|
|
this case, if the L2 guest RAM is quite large, it will require a large number
|
|
of mappings, which might exceed the VFIO limit set on the host. The default
|
|
value is 65536, which can simply be reached with a 256MiB sized RAM.
|
|
|
|
The way to solve both problems, the slowdown and the limit being exceeded, is
|
|
to reduce the amount of requests to describe those same large mappings. This
|
|
can be achieved by using 2MiB pages, known as huge pages. By seeing the guest
|
|
RAM as larger pages, and because the virtual IOMMU device supports it, the
|
|
guest will require less mappings, which will prevent the limit from being
|
|
exceeded, but also will take less time to process them on the host. That's
|
|
how using huge pages as much as possible can speed up VM boot time.
|
|
|
|
### Basic usage
|
|
|
|
Let's look at an example of how to run a guest with huge pages.
|
|
|
|
First, make sure your system has enough pages to cover the entire guest RAM:
|
|
```bash
|
|
# This example creates 4096 hugepages
|
|
echo 4096 > /proc/sys/vm/nr_hugepages
|
|
```
|
|
|
|
Next step is simply to create the VM. Two things are important, first we want
|
|
the VM RAM to be mapped on huge pages by backing it with `/dev/hugepages`. And
|
|
second thing, we need to create some huge pages in the guest itself so they can
|
|
be consumed.
|
|
|
|
```bash
|
|
./cloud-hypervisor \
|
|
--cpus boot=1 \
|
|
--memory size=8G,hugepages=on \
|
|
--disk path=focal-server-cloudimg-amd64.raw \
|
|
--kernel custom-vmlinux \
|
|
--cmdline "console=ttyS0 console=hvc0 root=/dev/vda1 rw hugepagesz=2M hugepages=2048" \
|
|
--net tap=,mac=,iommu=on
|
|
```
|
|
|
|
### Nested usage
|
|
|
|
Let's now look at the specific example of nested virtualization. In order to
|
|
reach optimized performances, the L2 guest also need to be mapped based on
|
|
huge pages. Here is how to achieve this, assuming the physical device you are
|
|
passing through is `0000:00:01.0`.
|
|
|
|
```bash
|
|
./cloud-hypervisor \
|
|
--cpus boot=1 \
|
|
--memory size=8G,hugepages=on \
|
|
--disk path=focal-server-cloudimg-amd64.raw \
|
|
--kernel custom-vmlinux \
|
|
--cmdline "console=ttyS0 console=hvc0 root=/dev/vda1 rw kvm-intel.nested=1 vfio_iommu_type1.allow_unsafe_interrupts rw hugepagesz=2M hugepages=2048" \
|
|
--device path=/sys/bus/pci/devices/0000:00:01.0,iommu=on
|
|
```
|
|
|
|
Once the L1 VM is running, unbind the device from the default driver in the
|
|
guest, and bind it to VFIO (it should appear as `0000:00:04.0`).
|
|
|
|
```bash
|
|
echo 0000:00:04.0 > /sys/bus/pci/devices/0000\:00\:04.0/driver/unbind
|
|
echo 8086 1502 > /sys/bus/pci/drivers/vfio-pci/new_id
|
|
```
|
|
|
|
Last thing is to start the L2 guest with the huge pages memory backend.
|
|
|
|
```bash
|
|
./cloud-hypervisor \
|
|
--cpus boot=1 \
|
|
--memory size=4G,hugepages=on \
|
|
--disk path=focal-server-cloudimg-amd64.raw \
|
|
--kernel custom-vmlinux \
|
|
--cmdline "console=ttyS0 console=hvc0 root=/dev/vda1 rw" \
|
|
--device path=/sys/bus/pci/devices/0000:00:04.0
|
|
```
|