On 2020/2/24 下午9:40, Michael S. Tsirkin wrote:

quoted

Subject: [PATCH] vhost: do not set VIRTIO_F_IOMMU_PLATFORM when IOMMU is not
  used

We enable device IOTLB unconditionally when VIRTIO_F_IOMMU_PLATFORM is
negotiated. This lead unnecessary IOTLB miss/update transactions when
IOMMU is used. This patch fixes this.

Signed-off-by: Jason Wang<jasowang@redhat.com>
---
  hw/net/virtio-net.c | 3 +++
  hw/virtio/vhost.c   | 4 +---
  2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 3627bb1717..0d50e8bd34 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c

@@ -879,6 +879,9 @@ static void virtio_net_set_features(VirtIODevice *vdev, uint64_t features)
          virtio_net_apply_guest_offloads(n);
      }
  
+    if (vdev->dma_as == &address_space_memory)
+        features &= ~(1ULL << VIRTIO_F_IOMMU_PLATFORM);
+
      for (i = 0;  i < n->max_queues; i++) {
          NetClientState *nc = qemu_get_subqueue(n->nic, i);

This pokes at acked features. I think they are also
guest visible ...

It's the acked features of vhost device, so I guess not?

E.g virtio_set_features_nocheck() did:

     val &= vdev->host_features;
     if (k->set_features) {
         k->set_features(vdev, val);
     }
     vdev->guest_features = val;

Thanks