[PATCH] virtio_mmio: Set DMA masks appropriately

[Robin Murphy]

On 10/01/17 13:15, Arnd Bergmann wrote:
> On Tuesday, January 10, 2017 12:26:01 PM CET Robin Murphy wrote:
>> @@ -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));
> 
> You don't seem to do anything different when 64-bit DMA is unsupported.
> How do you prevent the use of kernel buffers that are above the first 4G
> here?

That's the token "give up and rely on SWIOTLB/IOMMU" point, which as we
already know won't necessarily work very well (because it's already the
situation without this patch), but is still arguably better than
nothing. As I've just replied elsewhere, I personally hate this idiom,
but it's the done thing given the current DMA mask API.

>> +       else if (vm_dev->version == 1)
>> +               dma_set_coherent_mask(&pdev->dev, DMA_BIT_MASK(32 + PAGE_SHIFT));
> 
> Why is this limitation only for the coherent mask?

AIUI, the "32-bit pointers to pages" limitation of legacy virtio only
applies to the location of the vring itself, which is allocated via
dma_alloc_coherent - the descriptors themselves hold full 64-bit
addresses pointing at the actual data, which is mapped using streaming
DMA. It relies on the API guarantee that if we've managed to set a
64-bit streaming mask, then setting a smaller coherent mask cannot fail
(DMA-API-HOWTO.txt:257)

This is merely an amalgamation of the logic already in place for
virtio-pci, I just skimped on duplicating all the rationale (I know
there's a mail thread somewhere I could probably dig up).

Robin.