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vhost: correctly detect the enabling IOMMU

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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>
This commit is contained in:
Jason Wang 2021-08-04 11:48:03 +08:00 committed by Michael S. Tsirkin
parent 3d1e5d86fe
commit ae4003738f
1 changed files with 1 additions and 1 deletions
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2
hw/virtio/vhost.c
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@ -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);
}