On 10/01/17 14:54, Michael S. Tsirkin wrote:

On Tue, Jan 10, 2017 at 12:26:01PM +0000, Robin Murphy wrote:

quoted

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 <redacted>
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));

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

quoted

+	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.

quoted

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