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

Andy Lutomirski
2014-09-17T16:15:40Z

On Wed, Sep 17, 2014 at 9:09 AM, Ira W. Snyder <iws@ovro.caltech.edu> wrote:
> 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.

Thanks. I'll fix this in v6 if there is a v6.

--Andy

>
> 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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--
Andy Lutomirski
AMA Capital Management, LLC