[RFC 2/4] virtio: Override device's DMA OPS with virtio_direct_dma_ops selectively

[Anshuman Khandual]

On 07/20/2018 09:29 AM, Anshuman Khandual wrote:
> Now that virtio core always needs all virtio devices to have DMA OPS, we
> need to make sure that the structure it points is the right one. In the
> absence of VIRTIO_F_IOMMU_PLATFORM flag QEMU expects GPA from guest kernel.
> In such case, virtio device must use default virtio_direct_dma_ops DMA OPS
> structure which transforms scatter gather buffer addresses as GPA. This
> DMA OPS override must happen as early as possible during virtio device
> initializatin sequence before virtio core starts using given device's DMA
> OPS callbacks for I/O transactions. This change detects device's IOMMU flag
> and does the override in case the flag is cleared.
> 
> Signed-off-by: Anshuman Khandual <khandual at linux.vnet.ibm.com>
> ---
>  drivers/virtio/virtio.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index 7907ad3..6b13987 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -166,6 +166,8 @@ void virtio_add_status(struct virtio_device *dev, unsigned int status)
>  }
>  EXPORT_SYMBOL_GPL(virtio_add_status);
> 
> +const struct dma_map_ops virtio_direct_dma_ops;
> +
>  int virtio_finalize_features(struct virtio_device *dev)
>  {
>  	int ret = dev->config->finalize_features(dev);
> @@ -174,6 +176,9 @@ int virtio_finalize_features(struct virtio_device *dev)
>  	if (ret)
>  		return ret;


The previous patch removed the code block for XEN guests which forced
the use of DMA API all the time irrespective of VIRTIO_F_IOMMU_PLATFORM
flag on the device. Here is what I have removed with patch 2/4 which
breaks the existing semantics on XEN guests.

-static bool vring_use_dma_api(struct virtio_device *vdev)
-{
-	if (!virtio_has_iommu_quirk(vdev))
-		return true;
-
-	/* Otherwise, we are left to guess. */
-	/*
-	 * In theory, it's possible to have a buggy QEMU-supposed
-	 * emulated Q35 IOMMU and Xen enabled at the same time.  On
-	 * such a configuration, virtio has never worked and will
-	 * not work without an even larger kludge.  Instead, enable
-	 * the DMA API if we're a Xen guest, which at least allows
-	 * all of the sensible Xen configurations to work correctly.
-	 */
-	if (xen_domain())
-		return true;
-
-	return false;
-}

XEN guests would not like override with virtio_direct_dma_ops in any
case irrespective of the flag VIRTIO_F_IOMMU_PLATFORM. So the existing
semantics can be preserved with something like this. It just assumes
that dev->dma_ops is non-NULL and a valid one set by the architecture.
If required we can add those tests here before skipping the override.

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index 7907ad3..6b13987 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -166,6 +166,8 @@ void virtio_add_status(struct virtio_device *dev, unsigned int status)
 }
 EXPORT_SYMBOL_GPL(virtio_add_status);

+const struct dma_map_ops virtio_direct_dma_ops;
+
 int virtio_finalize_features(struct virtio_device *dev)
 {
 	int ret = dev->config->finalize_features(dev);
@@ -174,6 +176,9 @@ int virtio_finalize_features(struct virtio_device *dev)
 	if (ret)
 		return ret;
+
+	if (xen_domain())
+		goto skip_override;
+
+	if (virtio_has_iommu_quirk(dev))
+		set_dma_ops(dev->dev.parent, &virtio_direct_dma_ops);
+
+ skip_override:
+
 	if (!virtio_has_feature(dev, VIRTIO_F_VERSION_1))
 		return 0

Will incorporate these changes in the next version.