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

["Michael S. Tsirkin" <mst@redhat.com>]

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.


> 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?


> > 
> > 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;
>