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

Andy Lutomirski
Wed, 17 Sep 2014 16:07:17 -0000

On Sep 17, 2014 7:13 AM, "Michael S. Tsirkin" <mst at
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 at 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;
> >