Re: [PATCH v5 2/3] virtio_pci: Use the DMA API for virtqueues when possible

Ira W. Snyder
2014-09-17T16:09:45Z

On Tue, Sep 16, 2014 at 10:22:27PM -0700, Andy Lutomirski wrote:
> On non-PPC systems, virtio_pci should use the DMA API. This fixes
> virtio_pci on Xen. On PPC, using the DMA API would break things, so
> we need to preserve the old behavior.
>
> The big comment in this patch explains the considerations in more
> detail.
>
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
> drivers/virtio/virtio_pci.c | 90 ++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 81 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/virtio/virtio_pci.c b/drivers/virtio/virtio_pci.c
> index a1f299fa4626..8ddb0a641878 100644
> --- a/drivers/virtio/virtio_pci.c
> +++ b/drivers/virtio/virtio_pci.c
> @@ -80,8 +80,10 @@ 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 */
> + bool use_dma_api; /* are we using the DMA API? */
>
> /* the list node for the virtqueues list */
> struct list_head node;
> @@ -388,6 +390,50 @@ static int vp_request_intx(struct virtio_device *vdev)
> return err;
> }
>
> +static bool vp_use_dma_api(void)
> +{
> + /*
> + * Due to limitations of the DMA API, we only have two choices:
> + * use the DMA API (e.g. set up IOMMU mappings or apply Xen's
> + * physical-to-machine translation) or use direct physical
> + * addressing. Furthermore, there's no sensible way yet for the
> + * PCI bus code to tell us whether we're supposed to act like a
> + * normal PCI device (and use the DMA API) or to do something
> + * else. So we're stuck with heuristics here.
> + *
> + * In general, we would prefer to use the DMA API, since we
> + * might be driving a physical device, and such devices *must*
> + * use the DMA API if there is an IOMMU involved.
> + *
> + * On x86, there are no physically-mapped emulated virtio PCI
> + * devices that live behind an IOMMU. On ARM, there don't seem
> + * to be any hypervisors that use virtio_pci (as opposed to
> + * virtio_mmio) that also emulate an IOMMU. So using the DMI

Hi,

I noticed a typo here. It should say "DMA" not "DMI". Just thought I'd
point it out.

Ira

> + * API is safe.
> + *
> + * On PowerPC, it's the other way around. There usually is an
> + * IOMMU between us and the virtio PCI device, but the device is
> + * probably emulated and ignores the IOMMU. Unfortunately, we
> + * can't tell whether we're talking to an emulated device or to
> + * a physical device that really lives behind the IOMMU. That
> + * means that we're stuck with ignoring the DMA API.
> + */
> +
> +#ifdef CONFIG_PPC
> + return false;
> +#else
> + /*
> + * Minor 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 we already have a branch in
> + * the vring code, and we can avoid any further indirection with
> + * very little effort.
> + */
> + return !PCI_DMA_BUS_IS_PHYS;
> +#endif
> +}
> +
> static struct virtqueue *setup_vq(struct virtio_device *vdev, unsigned index,
> void (*callback)(struct virtqueue *vq),
> const char *name,
> @@ -416,21 +462,30 @@ static struct virtqueue *setup_vq(struct virtio_device *vdev, unsigned index,
>
> info->num = num;
> info->msix_vector = msix_vec;
> + info->use_dma_api = vp_use_dma_api();
>
> - 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);
> + if (info->use_dma_api) {
> + info->queue = dma_zalloc_coherent(vdev->dev.parent, size,
> + &info->queue_dma_addr,
> + GFP_KERNEL);
> + } else {
> + info->queue = alloc_pages_exact(PAGE_ALIGN(size),
> + GFP_KERNEL|__GFP_ZERO);
> + info->queue_dma_addr = virt_to_phys(info->queue);
> + }
> 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 */
> vq = vring_new_virtqueue(index, info->num, VIRTIO_PCI_VRING_ALIGN, vdev,
> - true, false, info->queue,
> + true, info->use_dma_api, info->queue,
> vp_notify, callback, name);
> if (!vq) {
> err = -ENOMEM;
> @@ -463,7 +518,12 @@ out_assign:
> vring_del_virtqueue(vq);
> out_activate_queue:
> iowrite32(0, vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN);
> - free_pages_exact(info->queue, size);
> + if (info->use_dma_api) {
> + dma_free_coherent(vdev->dev.parent, size,
> + info->queue, info->queue_dma_addr);
> + } else {
> + free_pages_exact(info->queue, PAGE_ALIGN(size));
> + }
> out_info:
> kfree(info);
> return ERR_PTR(err);
> @@ -493,8 +553,13 @@ static void vp_del_vq(struct virtqueue *vq)
> /* Select and deactivate the queue */
> 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);
> + size = vring_size(info->num, VIRTIO_PCI_VRING_ALIGN);
> + if (info->use_dma_api) {
> + dma_free_coherent(vq->vdev->dev.parent, size,
> + info->queue, info->queue_dma_addr);
> + } else {
> + free_pages_exact(info->queue, PAGE_ALIGN(size));
> + }
> kfree(info);
> }
>
> @@ -713,6 +778,13 @@ 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
>
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