[PATCH V3] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM

[Jason Wang <jasowang@redhat.com>]

We turn on device IOTLB via VIRTIO_F_IOMMU_PLATFORM unconditionally on
platform without IOMMU support. This can lead unnecessary IOTLB
transactions which will damage the performance.

Fixing this by check whether the device is backed by IOMMU and disable
device IOTLB.

Reported-by: Halil Pasic <pasic@linux.ibm.com>
Tested-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
Changes from V2:
- remove fixes tag
- tested with guest kernel virtio drivers + vIOMMU
Changes from V1:
- do not check acked_features
- reuse vhost_dev_has_iommu()
---
 hw/virtio/vhost.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 9edfadc81d..9182a00495 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -290,7 +290,14 @@ static int vhost_dev_has_iommu(struct vhost_dev *dev)
 {
     VirtIODevice *vdev = dev->vdev;
 
-    return virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
+    /*
+     * For vhost, VIRTIO_F_IOMMU_PLATFORM means the backend support
+     * incremental memory mapping API via IOTLB API. For platform that
+     * does not have IOMMU, there's no need to enable this feature
+     * which may cause unnecessary IOTLB miss/update trnasactions.
+     */
+    return vdev->dma_as != &address_space_memory &&
+           virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
 }
 
 static void *vhost_memory_map(struct vhost_dev *dev, hwaddr addr,
@@ -765,6 +772,9 @@ static int vhost_dev_set_features(struct vhost_dev *dev,
     if (enable_log) {
         features |= 0x1ULL << VHOST_F_LOG_ALL;
     }
+    if (!vhost_dev_has_iommu(dev)) {
+        features &= ~(0x1ULL << VIRTIO_F_IOMMU_PLATFORM);
+    }
     r = dev->vhost_ops->vhost_set_features(dev, features);
     if (r < 0) {
         VHOST_OPS_DEBUG("vhost_set_features failed");
-- 
2.19.1