[PATCH 1/2] drm/shmem: add support for per object dma api operations

[Thomas Zimmermann]

Hi

Am 12.06.20 um 20:54 schrieb Gurchetan Singh:
> On Fri, Jun 12, 2020 at 3:17 AM Gerd Hoffmann <kraxel at redhat.com> wrote:
>>
>> On Fri, Jun 12, 2020 at 11:47:55AM +0200, Thomas Zimmermann wrote:
>>> Hi
>>>
>>> Am 12.06.20 um 03:36 schrieb Gurchetan Singh:
>>>> This is useful for the next patch.  Also, should we only unmap the
>>>> amount entries that we mapped with the dma-api?
>>>
>>> It looks like you're moving virtio code into shmem.
>>
>> Well, not really.
>>
>> virtio has -- for historical reasons -- the oddity that it may or may
>> not need to dma_map resources, depending on device configuration.
>> Initially virtio went with "it's just a vm, lets simply operate on
>> physical ram addresses".  That shortcut turned out to be a bad idea
>> later on, especially with the arrival of iommu emulation support in
>> qemu.  But we couldn't just scratch it for backward compatibility
>> reasons.  See virtio_has_iommu_quirk().
>>
>> This just allows to enable/disable dma_map, I guess to fix some fallout
>> from recent shmem helper changes
> 
> Yes, the main goal is to fix the double free.
> 
> diff --git a/drivers/gpu/drm/virtio/virtgpu_object.c
> b/drivers/gpu/drm/virtio/virtgpu_object.c
> index 346cef5ce251..2f7b6cd25a4b 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_object.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_object.c
> @@ -78,7 +78,6 @@ void virtio_gpu_cleanup_object(struct virtio_gpu_object *bo)
>                                 shmem->mapped = 0;
>                         }
> 
> -                       sg_free_table(shmem->pages);
>                         shmem->pages = NULL;
>                         drm_gem_shmem_unpin(&bo->base.base);
>                 }
> 
> Doing only that on it's own causes log spam though
> 
> [   10.368385] virtio_gpu virtio5: swiotlb buffer is full (sz: 4096
> bytes), total 0 (slots), used 0 (slots)
> [   10.384957] virtio_gpu virtio5: swiotlb buffer is full (sz: 4096
> bytes), total 0 (slots), used 0 (slots)
> [   10.389920] virtio_gpu virtio5: swiotlb buffer is full (sz: 4096
> bytes), total 0 (slots), used 0 (slots)
> [   10.396859] virtio_gpu virtio5: swiotlb buffer is full (sz: 4096
> bytes), total 0 (slots), used 0 (slots)
> [   10.401954] virtio_gpu virtio5: swiotlb buffer is full (sz: 4096
> bytes), total 0 (slots), used 0 (slots)
> [   10.406694] virtio_gpu virtio5: swiotlb buffer is full (sz: 8192
> bytes), total 0 (slots), used 0 (slots)
> [   10.495744] virtio_gpu virtio5: swiotlb buffer is full (sz: 4096
> bytes), total 0 (slots), used 0 (slots)
> 
> Plus, I just realized the virtio dma ops and ops used by drm shmem are
> different, so virtio would have to unconditionally have to skip the
> shmem path.  Perhaps the best policy is to revert d323bb44e4d2?

Can you fork the shmem code into virtio and modify it according to your
needs? I know that code splitting is unpopular, but at least it's a
clean solution. For some GEM object functions, you might still be able
to share code with shmem helpers.

Best regards
Thomas

> 
>> (Gurchetan, that kind of stuff should
>> be mentioned in cover letter and commit messages).
> 
> Good tip.
> 
>>
>> I'm not sure virtio actually needs that patch though.  I *think* doing
>> the dma_map+dma_unmap unconditionally, but then ignore the result in
>> case we don't need it should work.  And it shouldn't be a horrible
>> performance hit either, in case we don't have a (virtual) iommu in the
>> VM dma mapping is essentially a nop ...
>>
>> take care,
>>   Gerd
>>

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

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