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vring: Introduce vring_use_dma_api()

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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
day.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
This commit is contained in:
Andy Lutomirski 2016-02-02 21:46:35 -08:00 committed by Michael S. Tsirkin
parent e82becfc18
commit d26c96c810
1 changed files with 24 additions and 0 deletions
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24
drivers/virtio/virtio_ring.c
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@ -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)
{