Re: [PATCH] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM

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

On Wed, Feb 26, 2020 at 03:06:47PM +0800, Jason Wang wrote:
> 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>
> Fixes: c471ad0e9bd46 ("vhost_net: device IOTLB support")
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  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..6e12c3d2de 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_has_feature(dev->acked_features, VIRTIO_F_IOMMU_PLATFORM);
>  }
>  
>  static void *vhost_memory_map(struct vhost_dev *dev, hwaddr addr,

Why check acked_features and not host features here?
I'd worry that if we do it like this, userspace driver
within guest can clear the feature and make device access
memory directly.

> @@ -765,6 +772,9 @@ static int vhost_dev_set_features(struct vhost_dev *dev,
>      if (enable_log) {
>          features |= 0x1ULL << VHOST_F_LOG_ALL;
>      }
> +    if (dev->vdev->dma_as == &address_space_memory) {
> +        features &= ~(0x1ULL << VIRTIO_F_IOMMU_PLATFORM);
> +    }


That's a guest visible change. Which seems at best unnecessary.

>      r = dev->vhost_ops->vhost_set_features(dev, features);
>      if (r < 0) {
>          VHOST_OPS_DEBUG("vhost_set_features failed");
> -- 
> 2.19.1