[PATCH] virtio_mmio: Set DMA masks appropriately

[Robin Murphy]

On 10/01/17 14:54, Michael S. Tsirkin wrote:
> 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 at arm.com>
>> Fixes: b42111382f0e ("virtio_mmio: Use the DMA API if enabled")
>> Signed-off-by: Robin Murphy <robin.murphy at 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));

Purely because it's fewer lines of code - if you'd prefer separate
legacy vs. modern flows for clarity that's fine by me.

>> +	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?

If we're not actually using the DMA API, we may still get away with it,
otherwise it's a fairly sure bet that the subsequent dma_map/dma_alloc
calls will fail and we'll get nowhere. If I change this to be a probe
failure condition (and correspondingly in the PCI drivers too), would
you rather that be predicated on vring_use_dma_api(), or always (given
the TODO in virtio_ring.c)?

Robin.

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