Hi libvirt experts,

Thanks for valuable input on v1 version of RFC.

Quick brief, VFIO based mediated device framework provides a way to
virtualize their devices without SR-IOV, like NVIDIA vGPU, Intel KVMGT
and IBM's channel IO. This framework reuses VFIO APIs for all the
functionalities for mediated devices which are currently being used for
pass through devices. This framework introduces a set of new sysfs files
for device creation and its life cycle management.

Here is the summary of discussion on v1:
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.

Vendor driver should specify mdev_supported_types in directory format.
This format is class based, for example, display class directory format
should be as below. We need to define such set for each class of devices
which would be supported by mediated device framework.

 --- mdev_destroy
 --- mdev_supported_types
     |-- 11
     |   |-- create
     |   |-- name
     |   |-- fb_length
     |   |-- resolution
     |   |-- heads
     |   |-- max_instances
     |   |-- params
     |   |-- requires_group
     |-- 12
     |   |-- create
     |   |-- name
     |   |-- fb_length
     |   |-- resolution
     |   |-- heads
     |   |-- max_instances
     |   |-- params
     |   |-- requires_group
     |-- 13
         |-- create
         |-- name
         |-- fb_length
         |-- resolution
         |-- heads
         |-- max_instances
         |-- params
         |-- requires_group

In the above example directory '11' represents a type id of mdev device.
'name', 'fb_length', 'resolution', 'heads', 'max_instance' and
'requires_group' would be Read-Only files that vendor would provide to
describe about that type.

    Write-only file. Mandatory.
    Accepts string to create mediated device.

    Read-Only file. Mandatory.
    Returns string, the name of that type id.

    Read-only file. Mandatory.
    Returns <number>{K,M,G}, size of framebuffer.

    Read-Only file. Mandatory.
    Returns 'hres x vres' format. Maximum supported resolution.

    Read-Only file. Mandatory.
    Returns integer. Number of maximum heads supported.

    Read-Only file. Mandatory.
    Returns integer.  Returns maximum mdev device could be created
at the moment when this file is read. This count would be updated by
vendor driver. Before creating mdev device of this type, check if
max_instance is > 0.

    Write-Only file. Optional.
    String input. Libvirt would pass the string given in XML file to
this file and then create mdev device. Set empty string to clear params.
For example, set parameter 'frame_rate_limiter=0' to disable frame rate
limiter for performance benchmarking, then create device of type 11. The
device created would have that parameter set by vendor driver.

    Read-Only file. Optional.
    This should be provided by vendor driver if vendor driver need to
group mdev devices in one domain so that vendor driver can use 'first
open' to commit resources of all mdev devices associated to that domain
and 'last close' to free those.

The parent device would look like:

     <capability type='pci'>
       <capability type='mdev'>
         <!-- one type element per sysfs directory -->
         <type id='11'>
           <!-- one element per sysfs file roughly -->
           <name>GRID M60-0B</name>
           <attribute name='fb_length'>512M</attribute>
           <attribute name='resolution'>2560x1600</attribute>
           <attribute name='heads'>2</attribute>
           <attribute name='max_instances'>16</attribute>
           <attribute name='requires_group'>1</attribute>
       <product id='...'>GRID M60</product>
       <vendor id='0x10de'>NVIDIA</vendor>

2. Create/destroy mediated device

With above example, vGPU device XML would look like:

     <capability type='mdev'>
       <type id='11'/>

'type id' is mandatory.
'group' is optional. It should be a unique number in the system among
all the groups created for mdev devices. Its usage is:
  - not needed if single vGPU device is being assigned to a domain.
  - only need to be set if multiple vGPUs need to be assigned to a
domain and vendor driver have 'requires_group' file in type id directory.
  - if type id directory include 'requires_group' and user tries to
assign multiple vGPUs to a domain without having <group> field in XML,
it will create single vGPU.

'params' is optional field. User should set this field if extra
parameters need to be set for a particular vGPU device. Libvirt don't
need to parse these params. These are meant for vendor driver.

Libvirt need to follow the sequence to create device:
* Read /sys/../0000\:86\:00.0/11/max_instances. If it is greater than 0,
then only proceed else fail.

* Set extra params if 'params' field exist in device XML and 'params'
file exist in type id directory

    echo "frame_rate_limiter=0" > /sys/../0000\:86\:00.0/11/params

* Autogenerate UUID
* Create device:

    echo "$UUID:<group>" > /sys/../0000\:86\:00.0/11/create

    where <group> is optional. Group should be unique number among all
the groups created for mdev devices.

* Clear params, if set earlier:

    echo "" > /sys/../0000\:86\:00.0/11/params

* To destroy device:

    echo $UUID > /sys/../0000\:86\:00.0/mdev_destroy

3. Start/stop mediated device

No change or requirement for libvirt as this will be handled by open()
and close() callbacks to vendor driver. In case of multiple devices and
'requires_group' set, this will be handled in 'first open()' and 'last
close()' on device in that group.

4. Launch QEMU/VM

 Pass the mdev sysfs path to QEMU as vfio-pci device.
 For above vGPU device example:

    -device vfio-pci,sysfsdev=/sys/bus/mdev/devices/$UUID

5. QEMU/VM Shutdown sequence

No change or requirement for libvirt.

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


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