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

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Date: Mon, 25 Mar 2024 19:38:34 -0700
From: Rob Clark <robdclark@...il.com>
To: Dominik Behr <dbehr@...omium.org>
Cc: dri-devel@...ts.freedesktop.org, Rob Clark <robdclark@...omium.org>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Sumit Semwal <sumit.semwal@...aro.org>, Christian König <christian.koenig@....com>, 
	Simon Ser <contact@...rsion.fr>, open list <linux-kernel@...r.kernel.org>, 
	"open list:DMA BUFFER SHARING FRAMEWORK:Keyword:bdma_(?:buf|fence|resv)b" <linux-media@...r.kernel.org>, 
	"moderated list:DMA BUFFER SHARING FRAMEWORK:Keyword:bdma_(?:buf|fence|resv)b" <linaro-mm-sig@...ts.linaro.org>
Subject: Re: [PATCH] drm/prime: Unbreak virtgpu dma-buf export

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@...omium.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@...il.com> wrote:
>>
>> From: Rob Clark <robdclark@...omium.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@...omium.org>
>> Fixes: 207395da5a97 ("drm/prime: reject DMA-BUF attach when get_sg_table is missing")
>> Signed-off-by: Rob Clark <robdclark@...omium.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
>>