Re: [PATCH] drm/prime: Unbreak virtgpu dma-buf export

From: Rob Clark <robdclark@gmail.com>
To: Dominik Behr <dbehr@chromium.org>
Cc: dri-devel@lists.freedesktop.org,
	"Rob Clark" <robdclark@chromium.org>,
	"Maarten Lankhorst" <maarten.lankhorst@linux.intel.com>,
	"Maxime Ripard" <mripard@kernel.org>,
	"Thomas Zimmermann" <tzimmermann@suse.de>,
	"David Airlie" <airlied@gmail.com>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Sumit Semwal" <sumit.semwal@linaro.org>,
	"Christian König" <christian.koenig@amd.com>,
	"Simon Ser" <contact@emersion.fr>,
	"open list" <linux-kernel@vger.kernel.org>,
	"open list:DMA BUFFER SHARING
	FRAMEWORK:Keyword:bdma_(?:buf|fence|resv)b"
	<linux-media@vger.kernel.org>,
	"moderated list:DMA BUFFER SHARING
	FRAMEWORK:Keyword:bdma_(?:buf|fence|resv)b"
	<linaro-mm-sig@lists.linaro.org>
Subject: Re: [PATCH] drm/prime: Unbreak virtgpu dma-buf export
Date: Mon, 25 Mar 2024 19:38:34 -0700	[thread overview]
Message-ID: <CAF6AEGvK7fBhh36rQyqiHf_qVvsCJhHZjoyOqGqxRPcMZ3dzjw@mail.gmail.com> (raw)
In-Reply-To: <CABUrSUAykXUoX0y6jzCw1s=E=E08Z54Ygo1TvBm49_iT88oDFg@mail.gmail.com>

This is actually a bit concerning.. importing a host page backed
buffer without guest mapping into a passthru device probably doesn't
work and should be rejected earlier.

I do think we should relax the restriction (either taking my patch or
reverting the commit it fixes) until we work this out properly
(because the original patch is a regression), but importing a buffer
without guest pages into a passthru device can't possibly work
properly.  Maybe it works by chance if the host buffer is mapped to
the guest, but that is not guaranteed.

BR,
-R

On Mon, Mar 25, 2024 at 3:35 PM Dominik Behr <dbehr@chromium.org> wrote:
>
> It also fixes importing virtgpu blobs into real hardware, for instance amdgpu for DRI_PRIME rendering.
>
> On Fri, Mar 22, 2024 at 2:48 PM Rob Clark <robdclark@gmail.com> wrote:
>>
>> From: Rob Clark <robdclark@chromium.org>
>>
>> virtgpu "vram" GEM objects do not implement obj->get_sg_table().  But
>> they also don't use drm_gem_map_dma_buf().  In fact they may not even
>> have guest visible pages.  But it is perfectly fine to export and share
>> with other virtual devices.
>>
>> Reported-by: Dominik Behr <dbehr@chromium.org>
>> Fixes: 207395da5a97 ("drm/prime: reject DMA-BUF attach when get_sg_table is missing")
>> Signed-off-by: Rob Clark <robdclark@chromium.org>
>> ---
>>  drivers/gpu/drm/drm_prime.c | 7 ++++++-
>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
>> index 7352bde299d5..64dd6276e828 100644
>> --- a/drivers/gpu/drm/drm_prime.c
>> +++ b/drivers/gpu/drm/drm_prime.c
>> @@ -582,7 +582,12 @@ int drm_gem_map_attach(struct dma_buf *dma_buf,
>>  {
>>         struct drm_gem_object *obj = dma_buf->priv;
>>
>> -       if (!obj->funcs->get_sg_table)
>> +       /*
>> +        * drm_gem_map_dma_buf() requires obj->get_sg_table(), but drivers
>> +        * that implement their own ->map_dma_buf() do not.
>> +        */
>> +       if ((dma_buf->ops->map_dma_buf == drm_gem_map_dma_buf) &&
>> +           !obj->funcs->get_sg_table)
>>                 return -ENOSYS;
>>
>>         return drm_gem_pin(obj);
>> --
>> 2.44.0
>>