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 10:54:54 +0100
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     Gerd Hoffmann <kraxel@...hat.com>
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.

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...

/Thomas


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