drm/prime: Unbreak virtgpu dma-buf export

  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(-)
  

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
>
  

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
>>
  

Makes sense to me!

Reviewed-by: Simon Ser <contact@emersion.fr>
  

It is mapped, via iommu interface (vfio-iommu) when map_dma is called.
--
Dominik

On Mon, Mar 25, 2024 at 7:38 PM Rob Clark <robdclark@gmail.com> wrote:
>
> 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
> >>
  

On Tue, Mar 26, 2024 at 2:15 AM Simon Ser <contact@emersion.fr> wrote:
>
> Makes sense to me!
>
> Reviewed-by: Simon Ser <contact@emersion.fr>

Thanks.. any chance I could talk you into pushing to drm-misc-fixes?

BR,
-R
  

On Thursday, March 28th, 2024 at 19:47, Rob Clark <robdclark@gmail.com> wrote:

> any chance I could talk you into pushing to drm-misc-fixes?

Oh sorry, I thought you had access… Pushed with a minor edit to remove
unnecessary parentheses to make checkpatch happy!
  

On Thu, Mar 28, 2024 at 11:54 AM Simon Ser <contact@emersion.fr> wrote:
>
> On Thursday, March 28th, 2024 at 19:47, Rob Clark <robdclark@gmail.com> wrote:
>
> > any chance I could talk you into pushing to drm-misc-fixes?
>
> Oh sorry, I thought you had access… Pushed with a minor edit to remove
> unnecessary parentheses to make checkpatch happy!

Thanks!

BR,
-R