Re: [PATCH 1/1] drm/virtio: Implement device_attach

[Zhang, Julia]

On 2024/1/10 18:21, Daniel Vetter wrote:
> On Wed, Jan 10, 2024 at 05:56:28PM +0800, Julia Zhang wrote:
>> drm_gem_map_attach() requires drm_gem_object_funcs.get_sg_table to be
>> implemented, or else return ENOSYS. Virtio has no get_sg_table
>> implemented for vram object. To fix this, add a new device_attach to
>> call drm_gem_map_attach() for shmem object and return 0 for vram object
>> instead of calling drm_gem_map_attach for both of these two kinds of
>> object.
>>
>> Signed-off-by: Julia Zhang <julia.zhang@xxxxxxx>
>> ---
>>  drivers/gpu/drm/virtio/virtgpu_prime.c | 14 +++++++++++++-
>>  1 file changed, 13 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/virtio/virtgpu_prime.c b/drivers/gpu/drm/virtio/virtgpu_prime.c
>> index 44425f20d91a..f0b0ff6f3813 100644
>> --- a/drivers/gpu/drm/virtio/virtgpu_prime.c
>> +++ b/drivers/gpu/drm/virtio/virtgpu_prime.c
>> @@ -71,6 +71,18 @@ static void virtgpu_gem_unmap_dma_buf(struct dma_buf_attachment *attach,
>>  	drm_gem_unmap_dma_buf(attach, sgt, dir);
>>  }
>>  
>> +static int virtgpu_gem_device_attach(struct dma_buf *dma_buf,
>> +				     struct dma_buf_attachment *attach)
>> +{
>> +	struct drm_gem_object *obj = attach->dmabuf->priv;
>> +	struct virtio_gpu_object *bo = gem_to_virtio_gpu_obj(obj);
>> +
>> +	if (virtio_gpu_is_vram(bo))
>> +		return 0;
> 
> You need to reject attach here because these vram buffer objects cannot be
> used by any other driver. In that case dma_buf_attach _must_ fail, not
> silently succeed.
> 

Do you mean these vram buffer objects should not be imported by other drivers?

> Because if it silently succeeds then the subsequent dma_buf_map_attachment
> will blow up because you don't have the ->get_sg_table hook implemented.
> 

I saw only this call stack would call ->get_sg_table:

#0 ->get_sg_table
#1 drm_gem_map_dma_buf
#2 virtgpu_gem_map_dma_buf
#3 __map_dma_buf
#4 dma_buf_dynamic_attach
#5 amdgpu_gem_prime_import

this stack is for shmem object and it requires ->get_sg_table get implemented.


But for vram object, __map_dma_buf will call like this:

#0 sg_alloc_table
#1 virtio_gpu_vram_map_dma_buf
#2 virtgpu_gem_map_dma_buf
#3 __map_dma_buf
#4 dma_buf_dynamic_attach
#5 amdgpu_gem_prime_import

which will not call ->get_sg_table but alloc a sg table instead and fill it from the vram object.

Before __map_dma_buf, the call stack of virtgpu_gem_device_attach is:

#0 virtgpu_gem_device_attach
#1 virtio_dma_buf_attach
#2 dma_buf_dynamic_attach
#3 amdgpu_gem_prime_import

So my problem is that to realize dGPU prime feature on VM, I actually want dma_buf_attach succeed
for vram object so that passthrough dGPU can import these vram objects and blit data to it. 
If here return -EBUSY, then amdgpu_gem_prime_import will fail and the whole feature will fail.

> Per the documentation the error code for this case must be -EBUSY, see the
> section for the attach hook here:
> 
> https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html#c.dma_buf_ops
> 
> Since you're looking into this area, please make sure there's not other
> similar mistake in virtio code.
> 
> Also can you please make a kerneldoc patch for struct virtio_dma_buf_ops
> to improve the documentation there? I think it would be good to move those
> to the inline style and then at least put a kernel-doc hyperlink to struct
> dma_buf_ops.attach and mention that attach must fail for non-shareable
> buffers.
> 
> In general the virtio_dma_buf kerneldoc seems to be on the "too minimal,
> explains nothing" side of things :-/

OK, let me take a look and try to do it.

Regards,
Julia

> 
> Cheers, Sima
> 
>> +
>> +	return drm_gem_map_attach(dma_buf, attach);
>> +}
>> +
>>  static const struct virtio_dma_buf_ops virtgpu_dmabuf_ops =  {
>>  	.ops = {
>>  		.cache_sgt_mapping = true,
>> @@ -83,7 +95,7 @@ static const struct virtio_dma_buf_ops virtgpu_dmabuf_ops =  {
>>  		.vmap = drm_gem_dmabuf_vmap,
>>  		.vunmap = drm_gem_dmabuf_vunmap,
>>  	},
>> -	.device_attach = drm_gem_map_attach,
>> +	.device_attach = virtgpu_gem_device_attach,
>>  	.get_uuid = virtgpu_virtio_get_uuid,
>>  };
>>  
>> -- 
>> 2.34.1
>>
>