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

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Date:   Fri, 28 Feb 2020 10:49:03 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     dri-devel@...ts.freedesktop.org, Guillaume.Gardet@....com,
        David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        gurchetansingh@...omium.org, tzimmermann@...e.de, yuq825@...il.com,
        noralf@...nnes.org, robh@...nel.org
Subject: Re: [PATCH v5 1/3] drm/shmem: add support for per object caching
 flags.

  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