[PULL 15/35] vhost: correctly detect the enabling IOMMU

["Michael S. Tsirkin" <mst@redhat.com>]

From: Jason Wang <jasowang@redhat.com>

Vhost used to compare the dma_as against the address_space_memory to
detect whether the IOMMU is enabled or not. This might not work well
since the virito-bus may call get_dma_as if VIRTIO_F_IOMMU_PLATFORM is
set without an actual IOMMU enabled when device is plugged. In the
case of PCI where pci_get_address_space() is used, the bus master as
is returned. So vhost actually tries to enable device IOTLB even if
the IOMMU is not enabled. This will lead a lots of unnecessary
transactions between vhost and Qemu and will introduce a huge drop of
the performance.

For PCI, an ideal approach is to use pci_device_iommu_address_space()
just for get_dma_as. But Qemu may choose to initialize the IOMMU after
the virtio-pci which lead a wrong address space is returned during
device plugged. So this patch switch to use transport specific way via
iommu_enabled() to detect the IOMMU during vhost start. In this case,
we are fine since we know the IOMMU is initialized correctly.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20210804034803.1644-4-jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 hw/virtio/vhost.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index e21e144510..b4b29413e6 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -315,7 +315,7 @@ static int vhost_dev_has_iommu(struct vhost_dev *dev)
      * 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 &&
+    return virtio_bus_device_iommu_enabled(vdev) &&
            virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
 }
 
-- 
MST