[PATCH v4 2/4] virtio_pci: Use the DMA API for virtqueues

Andy Lutomirski
2014-09-01T17:39:33Z

A virtqueue is a coherent DMA mapping. Use the DMA API for it.
This fixes virtio_pci on Xen.

As an optimization, this only enables asks virtio_ring to use the
DMA API if !PCI_DMA_BUS_IS_PHYS. Eventually, once the DMA API is
known to be efficient on all relevant architectures, this
optimization can be removed.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
drivers/virtio/virtio_pci.c | 40 +++++++++++++++++++++++++++++++---------
1 file changed, 31 insertions(+), 9 deletions(-)

diff --git a/drivers/virtio/virtio_pci.c b/drivers/virtio/virtio_pci.c
index a1f299fa4626..226b46b08727 100644
--- a/drivers/virtio/virtio_pci.c
+++ b/drivers/virtio/virtio_pci.c
@@ -80,8 +80,9 @@ struct virtio_pci_vq_info
/* the number of entries in the queue */
int num;

- /* the virtual address of the ring queue */
- void *queue;
+ /* the ring queue */
+ void *queue; /* virtual address */
+ dma_addr_t queue_dma_addr; /* bus address */

/* the list node for the virtqueues list */
struct list_head node;
@@ -417,20 +418,32 @@ static struct virtqueue *setup_vq(struct virtio_device *vdev, unsigned index,
info->num = num;
info->msix_vector = msix_vec;

- size = PAGE_ALIGN(vring_size(num, VIRTIO_PCI_VRING_ALIGN));
- info->queue = alloc_pages_exact(size, GFP_KERNEL|__GFP_ZERO);
+ size = vring_size(num, VIRTIO_PCI_VRING_ALIGN);
+ info->queue = dma_zalloc_coherent(vdev->dev.parent, size,
+ &info->queue_dma_addr, GFP_KERNEL);
if (info->queue == NULL) {
err = -ENOMEM;
goto out_info;
}

/* activate the queue */
- iowrite32(virt_to_phys(info->queue) >> VIRTIO_PCI_QUEUE_ADDR_SHIFT,
+ iowrite32(info->queue_dma_addr >> VIRTIO_PCI_QUEUE_ADDR_SHIFT,
vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN);

- /* create the vring */
+ /*
+ * Create the vring. If there is an IOMMU of any sort, including
+ * Xen paravirt's ersatz IOMMU, use it. If the host wants physical
+ * addresses instead of bus addresses, the host shouldn't expose
+ * an IOMMU.
+ *
+ * As an optimization, if the platform promises to have physical
+ * PCI DMA, we turn off DMA mapping in virtio_ring. If the
+ * platform's DMA API implementation is well optimized, this
+ * should have almost no effect, but that's a dangerous thing to
+ * rely on.
+ */
vq = vring_new_virtqueue(index, info->num, VIRTIO_PCI_VRING_ALIGN, vdev,
- true, false, info->queue,
+ true, !PCI_DMA_BUS_IS_PHYS, info->queue,
vp_notify, callback, name);
if (!vq) {
err = -ENOMEM;
@@ -463,7 +476,8 @@ out_assign:
vring_del_virtqueue(vq);
out_activate_queue:
iowrite32(0, vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN);
- free_pages_exact(info->queue, size);
+ dma_free_coherent(vdev->dev.parent, size,
+ info->queue, info->queue_dma_addr);
out_info:
kfree(info);
return ERR_PTR(err);
@@ -494,7 +508,8 @@ static void vp_del_vq(struct virtqueue *vq)
iowrite32(0, vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN);

size = PAGE_ALIGN(vring_size(info->num, VIRTIO_PCI_VRING_ALIGN));
- free_pages_exact(info->queue, size);
+ dma_free_coherent(vq->vdev->dev.parent, size,
+ info->queue, info->queue_dma_addr);
kfree(info);
}

@@ -713,6 +728,13 @@ static int virtio_pci_probe(struct pci_dev *pci_dev,
if (err)
goto out;

+ err = dma_set_mask_and_coherent(&pci_dev->dev, DMA_BIT_MASK(64));
+ if (err)
+ err = dma_set_mask_and_coherent(&pci_dev->dev,
+ DMA_BIT_MASK(32));
+ if (err)
+ dev_warn(&pci_dev->dev, "Failed to enable 64-bit or 32-bit DMA. Trying to continue, but this might not work.\n");
+
err = pci_request_regions(pci_dev, "virtio-pci");
if (err)
goto out_enable_device;
--
1.9.3