Re: [PATCH] virtio_mmio: Set DMA masks appropriately

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jan 10, 2017 at 12:26:01PM +0000, Robin Murphy wrote:
> Once DMA API usage is enabled, it becomes apparent that virtio-mmio is
> inadvertently relying on the default 32-bit DMA mask, which leads to
> problems like rapidly exhausting SWIOTLB bounce buffers.
> 
> Ensure that we set the appropriate 64-bit DMA mask whenever possible,
> with the coherent mask suitably limited for the legacy vring as per
> a0be1db4304f ("virtio_pci: Limit DMA mask to 44 bits for legacy virtio
> devices").
> 
> Reported-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
> Fixes: b42111382f0e ("virtio_mmio: Use the DMA API if enabled")
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
>  drivers/virtio/virtio_mmio.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
> index 48bfea91dbca..b5c5d49ca598 100644
> --- a/drivers/virtio/virtio_mmio.c
> +++ b/drivers/virtio/virtio_mmio.c
> @@ -59,6 +59,7 @@
>  #define pr_fmt(fmt) "virtio-mmio: " fmt
>  
>  #include <linux/acpi.h>
> +#include <linux/dma-mapping.h>
>  #include <linux/highmem.h>
>  #include <linux/interrupt.h>
>  #include <linux/io.h>
> @@ -497,6 +498,7 @@ static int virtio_mmio_probe(struct platform_device *pdev)
>  	struct virtio_mmio_device *vm_dev;
>  	struct resource *mem;
>  	unsigned long magic;
> +	int rc;
>  
>  	mem = platform_get_resource(pdev, IORESOURCE_MEM, 0);
>  	if (!mem)
> @@ -548,6 +550,14 @@ static int virtio_mmio_probe(struct platform_device *pdev)
>  	if (vm_dev->version == 1)
>  		writel(PAGE_SIZE, vm_dev->base + VIRTIO_MMIO_GUEST_PAGE_SIZE);
>  
> +	rc = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(64));
> +	if (rc)
> +		rc = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(32));
> +	else if (vm_dev->version == 1)
> +		dma_set_coherent_mask(&pdev->dev, DMA_BIT_MASK(32 + PAGE_SHIFT));

That's a very convoluted way to do this, for version 1 you
set coherent mask to 64 then override it.
why not

if (vm_dev->version == 1) {
	dma_set_mask
	dma_set_coherent_mask
} else {
	dma_set_mask_and_coherent
}

if (rc)
	dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(32));

> +	if (rc)
> +		dev_warn(&pdev->dev, "Failed to enable 64-bit or 32-bit DMA.  Trying to continue, but this might not work.\n");
> +

is there a chance it actually still might work?

>  	platform_set_drvdata(pdev, vm_dev);
>  
>  	return register_virtio_device(&vm_dev->vdev);
> -- 
> 2.10.2.dirty