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[v7,4/9] vring: Introduce vring_use_dma_api()

Message ID 14db226dda9e0d6af5d2bfab0f75ffddfb116bf9.1454477782.git.luto@kernel.org
State New, archived
Headers show

Commit Message

Andy Lutomirski Feb. 3, 2016, 5:46 a.m. UTC
This is a kludge, but no one has come up with a a better idea yet.
We'll introduce DMA API support guarded by vring_use_dma_api().
Eventually we may be able to return true on more and more systems,
and hopefully we can get rid of vring_use_dma_api() entirely some

Signed-off-by: Andy Lutomirski <luto@kernel.org>
 drivers/virtio/virtio_ring.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)
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diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index e12e385f7ac3..ab0be6c084f6 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -104,6 +104,30 @@  struct vring_virtqueue {
 #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
+ * The interaction between virtio and a possible IOMMU is a mess.
+ *
+ * On most systems with virtio, physical addresses match bus addresses,
+ * and it doesn't particularly matter whether we use the DMA API.
+ *
+ * On some systems, including Xen and any system with a physical device
+ * that speaks virtio behind a physical IOMMU, we must use the DMA API
+ * for virtio DMA to work at all.
+ *
+ * On other systems, including SPARC and PPC64, virtio-pci devices are
+ * enumerated as though they are behind an IOMMU, but the virtio host
+ * ignores the IOMMU, so we must either pretend that the IOMMU isn't
+ * there or somehow map everything as the identity.
+ *
+ * For the time being, we preserve historic behavior and bypass the DMA
+ * API.
+ */
+static bool vring_use_dma_api(struct virtio_device *vdev)
+	return false;
 static struct vring_desc *alloc_indirect(struct virtqueue *_vq,
 					 unsigned int total_sg, gfp_t gfp)