Re: [PATCH v5 1/3] drm/shmem: add support for per object caching flags.

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> > Not clue about the others (lima, tiny, panfrost, v3d).  Maybe they use
> > write-combine just because this is what they got by default from
> > drm_gem_mmap_obj().  Maybe they actually need that.  Trying to Cc:
> > maintainters (and drop stable@).

> > virtio-gpu needs it, otherwise the host can't show the virtual display.
> > cirrus bounces everything via blits to vram, so it should be ok without
> > decrypted.  I guess that implies we should make decrypted configurable.
> 
> Decrypted here is clearly incorrect and violates the SEV spec, regardless of
> a config option.
> 
> The only correct way is currently to use dma_alloc_coherent() and
> mmap_coherent() to allocate decrypted memory and then use the
> pgprot_decrypted flag.

Hmm, virtio-gpu uses the dma api to allow the host access the gem
object.  So I think I have to correct the statement above, if I
understands things correctly the dma api will use (properly allocated)
decrypted bounce buffers and the virtio-gpu shmem objects don't need
pgprot_decrypted mappings.

That leaves the question what to do about pgprot_writecombine().  Any
comments from the driver maintainers (see first paragraph)?

cheers,
  Gerd