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[Qemu-devel] [PATCH V2 RFC] fixup! virtio: convert to use DMA api

From: Michael S. Tsirkin
Subject: [Qemu-devel] [PATCH V2 RFC] fixup! virtio: convert to use DMA api
Date: Thu, 21 Apr 2016 16:43:45 +0300

This adds a flag to enable/disable bypassing the IOMMU by
virtio devices.

This is on top of patch
virtio: convert to use DMA api

Tested with patchset
virtio-pci: iommu support (note: bit number has been kept at 34
intentionally to match posted guest code. a non-RFC version will
renumber bits to be contigious).

changes from v1:
    drop PASSTHROUGH flag

The interaction between virtio and DMA API is messy.

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.

Add a feature bit to detect that: VIRTIO_F_IOMMU_PLATFORM.

If not there, we preserve historic behavior and bypass the DMA
API unless within Xen guest. This is actually required for
systems, including SPARC and PPC64, where 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.

Re: non-virtio devices.

It turns out that on old QEMU hosts, only emulated devices which were
part of QEMU use the IOMMU.  Should we want to bypass the IOMMU for such
devices *only*, it would be rather easy to detect them by looking at
subsystem vendor and device ID. Thus, no new interfaces are required
except for virtio which always uses the same subsystem vendor and device ID.

Signed-off-by: Michael S. Tsirkin <address@hidden>
 include/hw/virtio/virtio-access.h              | 3 ++-
 include/hw/virtio/virtio.h                     | 4 +++-
 include/standard-headers/linux/virtio_config.h | 2 ++
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/hw/virtio/virtio-access.h 
index 967cc75..bb6f34e 100644
--- a/include/hw/virtio/virtio-access.h
+++ b/include/hw/virtio/virtio-access.h
@@ -23,7 +23,8 @@ static inline AddressSpace *virtio_get_dma_as(VirtIODevice 
     BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
     VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
-    if (k->get_dma_as) {
+    if (virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM) &&
+        k->get_dma_as) {
         return k->get_dma_as(qbus->parent);
     return &address_space_memory;
diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index b12faa9..44f3788 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -228,7 +228,9 @@ typedef struct VirtIORNGConf VirtIORNGConf;
     DEFINE_PROP_BIT64("notify_on_empty", _state, _field,  \
                       VIRTIO_F_NOTIFY_ON_EMPTY, true), \
     DEFINE_PROP_BIT64("any_layout", _state, _field, \
-                      VIRTIO_F_ANY_LAYOUT, true)
+                      VIRTIO_F_ANY_LAYOUT, true), \
+    DEFINE_PROP_BIT64("iommu_platform", _state, _field, \
+                      VIRTIO_F_IOMMU_PLATFORM, false)
 hwaddr virtio_queue_get_desc_addr(VirtIODevice *vdev, int n);
 hwaddr virtio_queue_get_avail_addr(VirtIODevice *vdev, int n);
diff --git a/include/standard-headers/linux/virtio_config.h 
index bcc445b..3fcfbb1 100644
--- a/include/standard-headers/linux/virtio_config.h
+++ b/include/standard-headers/linux/virtio_config.h
@@ -61,4 +61,6 @@
 /* v1.0 compliant. */
 #define VIRTIO_F_VERSION_1             32
+/* Do not bypass the IOMMU (if configured) */
+#define VIRTIO_F_IOMMU_PLATFORM                34
 #endif /* _LINUX_VIRTIO_CONFIG_H */

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