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

Andy Lutomirski
2014-09-17T16:07:17Z

On Sep 17, 2014 7:13 AM, "Michael S. Tsirkin" <mst@redhat.com> wrote:
>
> On Wed, Sep 17, 2014 at 08:02:31AM -0400, Benjamin Herrenschmidt wrote:
> > On Tue, 2014-09-16 at 22:22 -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.
> >
> > I still disagree with using CONFIG_PPC as a trigger here.
> >
> > Fundamentally, the qemu implementation today bypasses IOMMUs on all
> > platforms as far as I can tell.
> >
> > If that changes, we'll have a backward compatibility problem.
> >
> > The virtio device should advertise whether it's using that bypass
> > mode of operation and virtio_pci should react accordingly.
>
> Well if there's a way to detect that - that's outside the
> device, then we probably shouldn't use device-specific
> interfaces for this capability.

I still think that this is a property of the bus, not the device. x86
has such a mechanism, and this patch uses it transparently.

(NB: According to David Woodhouse about ten minutes ago, this should
work with a bit of care right now on x86 via ACPI as long as QEMU
configures the tables correctly.)

>
>
> > There is a demand for being able to operate on top of an IOMMU on
> > powerpc as well for some embedded stuff using PCI as a transport so your
> > patch precludes that.
> >
> > Cheers,
> > Ben.
>
> As far as I can see, nothing changes on PPC so this patch
> does not preclude anything that was working?
>

It shouldn't. That being said, at some point this problem will need
solving on PPC, and this patch doesn't help much, other than adding
the virtio_ring piece.

I'd really like to see the generic or arch IOMMU code handle this so
that the PPC special case in the virtio-pci driver can go away.

Another reason that this kind of special casing doesn't belong in
virtio-pci: if anyone ever binds vfio to a virtio-pci device, if the
DMA API doesn't work right, then the kernel will go boom or be
compromised.

--Andy

>
> > >
> > > 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
> > > + * 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;
> >