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[libvirt] [RFC] libvirt vGPU QEMU integration



Hi libvirt experts,

I am starting this email thread to discuss the potential solution / proposal of
integrating vGPU support into libvirt for QEMU.

Some quick background, NVIDIA is implementing a VFIO based mediated device
framework to allow people to virtualize their devices without SR-IOV, for
example NVIDIA vGPU, and Intel KVMGT. Within this framework, we are reusing the
VFIO API to process the memory / interrupt as what QEMU does today with passthru
device.

The difference here is that we are introducing a set of new sysfs file for
virtual device discovery and life cycle management due to its virtual nature.

Here is the summary of the sysfs, when they will be created and how they should
be used:

1. Discover mediated device

As part of physical device initialization process, vendor driver will register
their physical devices, which will be used to create virtual device (mediated
device, aka mdev) to the mediated framework.

Then, the sysfs file "mdev_supported_types" will be available under the physical
device sysfs, and it will indicate the supported mdev and configuration for this 
particular physical device, and the content may change dynamically based on the
system's current configurations, so libvirt needs to query this file every time
before create a mdev.

Note: different vendors might have their own specific configuration sysfs as
well, if they don't have pre-defined types.

For example, we have a NVIDIA Tesla M60 on 86:00.0 here registered, and here is
NVIDIA specific configuration on an idle system.

For example, to query the "mdev_supported_types" on this Tesla M60:

cat /sys/bus/pci/devices/0000:86:00.0/mdev_supported_types
# vgpu_type_id, vgpu_type, max_instance, num_heads, frl_config, framebuffer,
max_resolution
11      ,"GRID M60-0B",      16,       2,      45,     512M,    2560x1600
12      ,"GRID M60-0Q",      16,       2,      60,     512M,    2560x1600
13      ,"GRID M60-1B",       8,       2,      45,    1024M,    2560x1600
14      ,"GRID M60-1Q",       8,       2,      60,    1024M,    2560x1600
15      ,"GRID M60-2B",       4,       2,      45,    2048M,    2560x1600
16      ,"GRID M60-2Q",       4,       4,      60,    2048M,    2560x1600
17      ,"GRID M60-4Q",       2,       4,      60,    4096M,    3840x2160
18      ,"GRID M60-8Q",       1,       4,      60,    8192M,    3840x2160

2. Create/destroy mediated device

Two sysfs files are available under the physical device sysfs path : mdev_create
and mdev_destroy

The syntax of creating a mdev is:

    echo "$mdev_UUID:vendor_specific_argument_list" >
/sys/bus/pci/devices/.../mdev_create

The syntax of destroying a mdev is:

    echo "$mdev_UUID:vendor_specific_argument_list" >
/sys/bus/pci/devices/.../mdev_destroy

The $mdev_UUID is a unique identifier for this mdev device to be created, and it
is unique per system.

For NVIDIA vGPU, we require a vGPU type identifier (shown as vgpu_type_id in
above Tesla M60 output), and a VM UUID to be passed as
"vendor_specific_argument_list".

If there is no vendor specific arguments required, either "$mdev_UUID" or
"$mdev_UUID:" will be acceptable as input syntax for the above two commands.

To create a M60-4Q device, libvirt needs to do:

    echo "$mdev_UUID:vgpu_type_id=20,vm_uuid=$VM_UUID" >
/sys/bus/pci/devices/0000\:86\:00.0/mdev_create

Then, you will see a virtual device shows up at:

    /sys/bus/mdev/devices/$mdev_UUID/

For NVIDIA, to create multiple virtual devices per VM, it has to be created
upfront before bringing any of them online.

Regarding error reporting and detection, on failure, write() to sysfs using fd
returns error code, and write to sysfs file through command prompt shows the
string corresponding to error code.

3. Start/stop mediated device

Under the virtual device sysfs, you will see a new "online" sysfs file.

you can do cat /sys/bus/mdev/devices/$mdev_UUID/online to get the current status
of this virtual device (0 or 1), and to start a virtual device or stop a virtual 
device you can do:

    echo "1|0" > /sys/bus/mdev/devices/$mdev_UUID/online

libvirt needs to query the current state before changing state.

Note: if you have multiple devices, you need to write to the "online" file
individually.

For NVIDIA, if there are multiple mdev per VM, libvirt needs to bring all of
them "online" before starting QEMU.

4. Launch QEMU/VM

Pass the mdev sysfs path to QEMU as vfio-pci device:

    -device vfio-pci,sysfsdev=/sys/bus/mdev/devices/$mdev_UUID,id=vgpu0

5. Shutdown sequence 

libvirt needs to shutdown the qemu, bring the virtual device offline, then destroy the
virtual device

6. VM Reset

No change or requirement for libvirt as this will be handled via VFIO reset API
and QEMU process will keep running as before.

7. Hot-plug

It optional for vendors to support hot-plug.

And it is same syntax to create a virtual device for hot-plug. 

For hot-unplug, after executing QEMU monitor "device del" command, libvirt needs
to write to "destroy" sysfs to complete hot-unplug process.

Since hot-plug is optional, then mdev_create or mdev_destroy operations may
return an error if it is not supported.

Thanks, 
Neo


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