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

Zhenyu Wang
2018-10-11T01:14:22Z

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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.

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