Re: [Intel-gfx] refactor the i915 GVT support

Daniel Vetter
2021-07-29T07:30:25Z

On Thu, Jul 29, 2021 at 09:20:22AM +0200, Christoph Hellwig wrote:
> On Wed, Jul 28, 2021 at 02:59:25PM -0300, Jason Gunthorpe wrote:
> > On Wed, Jul 28, 2021 at 01:38:58PM +0000, Wang, Zhi A wrote:
> >
> > > I guess those APIs you were talking about are KVM-only. For other
> > > hypervisors, e.g. Xen, ARCN cannot use the APIs you mentioned. Not
> > > sure if you have already noticed that VFIO is KVM-only right now.
> >
> > There is very little hard connection between VFIO and KVM, so no, I
> > don't think that is completely true.
>
> The only connection is the SET_KVM notifier as far as I can tell.
> Which is used by a total of two drivers, including i915/gvt. That
> being said gvt does not only use vfio, but also does quite a few
> direct cals to KVM.
>
> > In an event, an in-tree version of other hypervisor support for GVT
> > needs to go through enabling VFIO support so that the existing API
> > multiplexers we have can be used properly, not adding a shim layer
> > trying to recreate VFIO inside a GPU driver.
>
> Yes. And if we go back to actually looking at the series a lot of
> it just removes entirely pointless indirect calls that go to generic
> code and not even the kvm code, or questionable data structure designs.
> If we were to support another upstream hypervisor we'd just need to
> union a few fields in struct intel_gpu and maybe introduce a few
> methods. Preferably in a way that avoids expensive indirect calls
> in the fast path.

fwiw I concur with the direction of this series. gvt landed 5 years ago,
that should have been plenty of time to merge at least one of the other
backends that float around. If it didn't happen in 5 years it aint
suddenly happening in the next few, and the abstraction layer should be
sunset.

Also yes structuring it more as a helper layer with some
unions/subclassing than full blown backend abstractor layer would be a
good idea too I guess (it usually is the right thing to do).
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation