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

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 11 Oct 2018 09:14:22 +0800
Zhenyu Wang <zhenyuw@linux.intel.com> wrote:

> On 2018.10.10 23:22:20 +0000, Tian, Kevin wrote:
> > > From: Alex Williamson
> > > Sent: Thursday, October 11, 2018 4:39 AM
> > > 
> > > On Tue, 9 Oct 2018 01:40:17 +0530
> > > Kirti Wankhede <kwankhede@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@nvidia.com>
> > > > Reviewed-by: Neo Jia <cjia@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@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?
> >  
> 
> yeah, as current intel gpu is single device, even guest i915 driver could
> operate but user space gfx driver is not ready for multiple devices, so won't
> be useful now.

The restriction sounds different here though, in the case of NVIDIA
it's the mdev vendor driver with the restriction, with Intel I think
I'm hearing that it's the guest graphics driver that's the
restriction.  I don't think we'd want to place an artificial
restriction at the mdev layer for a guest driver deficiency that could
theoretically be fixed independently from the host mdev vendor driver.
Thanks,

Alex