[libvirt] [PATCH v2 1/1] Add attribute single_usage_restriction for mdev type-id

[Tian, Kevin]

> From: Alex Williamson
> Sent: Thursday, October 11, 2018 4:39 AM
> 
> On Tue, 9 Oct 2018 01:40:17 +0530
> Kirti Wankhede <kwankhede at nvidia.com> wrote:
> 
> > Generally a single instance of mdev device, a share of physical device, is
> > assigned to user space application or a VM. There are cases when
> multiple
> > instances of mdev devices of same or different types are required by user
> > space application or VM. For example in case of vGPU, multiple mdev
> devices
> > of type which represents whole GPU can be assigned to one instance of
> > application or VM.
> >
> > All types of mdev devices may not support assigning multiple mdev
> devices
> > to a user space application. In that case vendor driver can fail open()
> > call of mdev device. But there is no way to know User space application to
> > about the configuration supported by vendor driver.
> >
> > To expose supported configuration, vendor driver should add
> > 'single_usage_restriction' attribute to type-id directory. Returning Y for
> > this attribute indicates vendor driver has restriction of single mdev
> > device of particular <type-id> assigned to one user space application.
> > Returning N indicates that multiple mdev devices of particular <type-id>
> > can be assigned to one user space application.
> >
> > User space application should read if 'single_usage_restriction' attibute
> > is present in <type-id> directory of all mdev devices which are going to be
> > used. If all read N then user space application can proceed with multiple
> > mdev devices.
> >
> > This is optional and readonly attribute.
> >
> > Signed-off-by: Kirti Wankhede <kwankhede at nvidia.com>
> > Reviewed-by: Neo Jia <cjia at nvidia.com>
> > ---
> >  Documentation/ABI/testing/sysfs-bus-vfio-mdev | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> >
> > diff --git a/Documentation/ABI/testing/sysfs-bus-vfio-mdev
> b/Documentation/ABI/testing/sysfs-bus-vfio-mdev
> > index 452dbe39270e..3aca352a70e5 100644
> > --- a/Documentation/ABI/testing/sysfs-bus-vfio-mdev
> > +++ b/Documentation/ABI/testing/sysfs-bus-vfio-mdev
> > @@ -85,6 +85,22 @@ Users:
> >  		a particular <type-id> that can help in understanding the
> >  		features provided by that type of mediated device.
> >
> > +What:		/sys/.../mdev_supported_types/<type-
> id>/single_usage_restriction
> > +Date:           October 2018
> > +Contact:        Kirti Wankhede <kwankhede at nvidia.com>
> > +Description:
> > +		Reading this attribute will return Y or N. Returning Y
> indicates
> > +		vendor driver has restriction of single mdev device of this
> > +		particular <type-id> assigned to one user space application.
> > +		Returning N indicates that multiple mdev devices of
> particular
> > +		<type-id> can be assigned to one user space application.
> > +		This is optional and readonly attribute.
> > +Users:
> > +		User space application should read if
> 'single_usage_restriction'
> > +		attibute is present in <type-id> directory of all mdev devices
> > +		which are going to be used. If all read N then user space
> > +		application can proceed with multiple mdev devices.
> 
> But we don't say what userspace should do when this optional attribute
> is not present.  Do we know of any cases other than the NVIDIA GRID
> vGPU drivers that have this restriction?  Intel folks, are multiple
> GVT-g mdevs currently allowed in a VM?  I don't think the libvirt
> algorithm is going to be as simple as suggested here and we should
> probably understand what it really needs to be.

technically I don't see a restriction in GVT-g side, i.e. multiple GVT-g
mdevs can be assigned to same VM. But the fact is that Intel GPU is 
integrated thus just one per platform. Then guest i915 driver may have 
problem to operate multiple vGPUs if with some assumption on integrated
part. I don't think we verified such configuration. Zhenyu?

> 
> There's also a scope issue that's unclear here, the verbiage above
> suggests that I can't combine a 'single_usage_restriction=Y' mdev with
> any other mdev, but clearly there's no dependency between adding both
> an NVIDIA and Intel vGPU to the same VM, right?  Or NVIDIA and any of
> the mdev sample drivers.  The restriction is across mdev types and
> parent devices in this case, but I think it stops at the vendor driver,
> right?  How do we make that more clear, both in wording and perhaps
> implicit in the attribute itself?  Thanks,
> 
> Alex