[PATCH] virtio: turn on IOMMU_PLATFORM properly

[Jason Wang]

When transport does not support IOMMU, we should clear IOMMU_PLATFORM
even if the device and vhost claims to support that. This help to
avoid the performance overhead caused by unnecessary IOTLB miss/update
transactions on such platform.

Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
---
 hw/virtio/virtio-bus.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
index d6332d45c3..2741b9fdd2 100644
--- a/hw/virtio/virtio-bus.c
+++ b/hw/virtio/virtio-bus.c
@@ -47,7 +47,6 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
     VirtioBusState *bus = VIRTIO_BUS(qbus);
     VirtioBusClass *klass = VIRTIO_BUS_GET_CLASS(bus);
     VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
-    bool has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
     Error *local_err = NULL;
 
     DPRINTF("%s: plug device.\n", qbus->name);
@@ -77,10 +76,11 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
         return;
     }
 
-    if (klass->get_dma_as != NULL && has_iommu) {
-        virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
+    if (false && klass->get_dma_as != NULL &&
+        virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
         vdev->dma_as = klass->get_dma_as(qbus->parent);
     } else {
+        virtio_clear_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
         vdev->dma_as = &address_space_memory;
     }
 }
-- 
2.19.1


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