Re: [PATCH] virtio_ring: Fix querying of maximum DMA mapping size for virtio device

[Suzuki K Poulose]

On 01/12/2021 11:20, Will Deacon wrote:
> virtio_max_dma_size() returns the maximum DMA mapping size of the virtio
> device by querying dma_max_mapping_size() for the device when the DMA
> API is in use for the vring. Unfortunately, the device passed is
> initialised by register_virtio_device() and does not inherit the DMA
> configuration from its parent, resulting in SWIOTLB errors when bouncing
> is enabled and the default 256K mapping limit (IO_TLB_SEGSIZE) is not
> respected:
>
>    | virtio-pci 0000:00:01.0: swiotlb buffer is full (sz: 294912 bytes), total 1024 (slots), used 725 (slots)
>
> Follow the pattern used elsewhere in the virtio_ring code when calling
> into the DMA layer and pass the parent device to dma_max_mapping_size()
> instead.
>
> Cc: Marc Zyngier <maz@xxxxxxxxxx>
> Cc: Quentin Perret <qperret@xxxxxxxxxx>
> Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> Cc: Jason Wang <jasowang@xxxxxxxxxx>
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>

We hit a similar problem and ended up in the same patch, a bit late [0].

FWIW:

Tested-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>

[0] 
https://lkml.kernel.org/r/20211203121614.3380162-1-suzuki.poulose@xxxxxxx


> ---
>   drivers/virtio/virtio_ring.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 6d2614e34470..028b05d44546 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -268,7 +268,7 @@ size_t virtio_max_dma_size(struct virtio_device *vdev)
>   	size_t max_segment_size = SIZE_MAX;
>   
>   	if (vring_use_dma_api(vdev))
> -		max_segment_size = dma_max_mapping_size(&vdev->dev);
> +		max_segment_size = dma_max_mapping_size(vdev->dev.parent);
>   
>   	return max_segment_size;
>   }