Re: [PATCH v3 6/7] vfio: Wait for dma-buf invalidation to complete

[Christian König]

On 1/21/26 10:36, Thomas Hellström wrote:
> Hi, Christian,
> 
> On Wed, 2026-01-21 at 10:20 +0100, Christian König wrote:
>> On 1/20/26 15:07, Leon Romanovsky wrote:
>>> From: Leon Romanovsky <leonro@xxxxxxxxxx>
>>>
>>> dma-buf invalidation is performed asynchronously by hardware, so
>>> VFIO must
>>> wait until all affected objects have been fully invalidated.
>>>
>>> Fixes: 5d74781ebc86 ("vfio/pci: Add dma-buf export support for MMIO
>>> regions")
>>> Signed-off-by: Leon Romanovsky <leonro@xxxxxxxxxx>
>>
>> Reviewed-by: Christian König <christian.koenig@xxxxxxx>
>>
>> Please also keep in mind that the while this wait for all fences for
>> correctness you also need to keep the mapping valid until
>> dma_buf_unmap_attachment() was called.
> 
> I'm wondering shouldn't we require DMA_RESV_USAGE_BOOKKEEP here, as
> *any* unsignaled fence could indicate access through the map?

Yes, exactly that. I totally missed this detail.

Thanks a lot to Matthew and you to pointing this out.

Regards,
Christian.

> 
> /Thomas
> 
>>
>> In other words you can only redirect the DMA-addresses previously
>> given out into nirvana (or a dummy memory or similar), but you still
>> need to avoid re-using them for something else.
>>
>> Regards,
>> Christian.
>>
>>> ---
>>>  drivers/vfio/pci/vfio_pci_dmabuf.c | 5 +++++
>>>  1 file changed, 5 insertions(+)
>>>
>>> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c
>>> b/drivers/vfio/pci/vfio_pci_dmabuf.c
>>> index d4d0f7d08c53..33bc6a1909dd 100644
>>> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
>>> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
>>> @@ -321,6 +321,9 @@ void vfio_pci_dma_buf_move(struct
>>> vfio_pci_core_device *vdev, bool revoked)
>>>  			dma_resv_lock(priv->dmabuf->resv, NULL);
>>>  			priv->revoked = revoked;
>>>  			dma_buf_move_notify(priv->dmabuf);
>>> +			dma_resv_wait_timeout(priv->dmabuf->resv,
>>> +					     
>>> DMA_RESV_USAGE_KERNEL, false,
>>> +					     
>>> MAX_SCHEDULE_TIMEOUT);
>>>  			dma_resv_unlock(priv->dmabuf->resv);
>>>  		}
>>>  		fput(priv->dmabuf->file);
>>> @@ -342,6 +345,8 @@ void vfio_pci_dma_buf_cleanup(struct
>>> vfio_pci_core_device *vdev)
>>>  		priv->vdev = NULL;
>>>  		priv->revoked = true;
>>>  		dma_buf_move_notify(priv->dmabuf);
>>> +		dma_resv_wait_timeout(priv->dmabuf->resv,
>>> DMA_RESV_USAGE_KERNEL,
>>> +				      false,
>>> MAX_SCHEDULE_TIMEOUT);
>>>  		dma_resv_unlock(priv->dmabuf->resv);
>>>  		vfio_device_put_registration(&vdev->vdev);
>>>  		fput(priv->dmabuf->file);
>>>