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

[Gerd Hoffmann <kraxel@redhat.com>]

On Fri, Feb 28, 2020 at 10:54:54AM +0100, Thomas Hellström (VMware) wrote:
> On 2/28/20 10:49 AM, Gerd Hoffmann wrote:
> >    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.
> 
> Yes, that sounds more correct. I wonder whether the "pgprot_decrypted()"
> perhaps remains from mapping VRAM gem buffers...

Commit 95cf9264d5f3 ("x86, drm, fbdev: Do not specify encrypted memory
for video mappings") added it, then it was kept through various changes.

cheers,
  Gerd