Re: [virtio-dev] Re: [PATCH] virtio-gpu: add shared resource feature

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> > They are never taken away from guest ram.  The guest is free to do
> > whatever it wants with the pages.  It's the job of the guest driver to
> > make sure the pages are not released until the host stopped using them.
> > So the host must drop all references before completing the
> > DETACH_BACKING command.
> 
> Let's write that in the spec.

Yes, saying so explicitly that totally makes sense.
I'll update the patch accordingly.

> We (you?) need to modify drm_gem_shmem_* helpers to always be cached
> for virtgpu for now, since the TTM removal changed that behavior.

We can just roll our own mmap function to handle that.
I'll prepare a patch (that is independent from F_SHARED anyway).

> Then, we should expose host properties to the guest, like [1] suggests
> (maybe something like VIRTIO_GPU_F_GUEST_ATTRIBUTE_OVERRIDE,
> VIRTIO_GPU_F_GUEST_CACHE_OPS).  If there's already a method to do
> this, that would be awesome.

We'll need that for sure when mapping host resources into the guest, but
we are not there yet with F_SHARED only.

> > Also: what is special in virtualization here?  There are arm drivers
> > using drm_gem_shmem_* helpers.  How do they manage this?
> 
> Panfrost, v3d.  I'm surprised they haven't hit this issue yet.

Maybe because all mappings are consistent.  With virtio-gpu you could
end up in a situation where the guest uses wc whereas the host uses
cached and things go boom from there.

cheers,
  Gerd