Re: [PATCH] hw/virtio/vhost: Disable IOTLB callbacks when IOMMU gets disabled

[Jason Wang]

On Tue, Jan 21, 2025 at 1:33 AM Eric Auger <eric.auger@redhat.com> wrote:
>
> When a guest exposed with a vhost device and protected by an
> intel IOMMU gets rebooted, we sometimes observe a spurious warning:
>
> Fail to lookup the translated address ffffe000
>
> We observe that the IOMMU gets disabled through a write to the global
> command register (CMAR_GCMD.TE) before the vhost device gets stopped.
> When this warning happens it can be observed an inflight IOTLB
> miss occurs after the IOMMU disable and before the vhost stop. In
> that case a flat translation occurs and the check in
> vhost_memory_region_lookup() fails.
>
> Let's disable the IOTLB callbacks when all IOMMU MRs have been
> unregistered.
>
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> ---
>  hw/virtio/vhost.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index 6aa72fd434..128c2ab094 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -931,6 +931,10 @@ static void vhost_iommu_region_del(MemoryListener 
> *listener,
>              break;
>          }
>      }
> +    if (QLIST_EMPTY(&dev->iommu_list) &&
> +        dev->vhost_ops->vhost_set_iotlb_callback) {
> +        dev->vhost_ops->vhost_set_iotlb_callback(dev, false);
> +    }

So the current code assumes:

1) IOMMU is enabled before vhost starts
2) IOMMU is disabled after vhost stops

This patch seems to fix 2) but not 1). Do we need to deal with the
IOMMU enabled after vhost starts?

Thanks

>  }
>
>  void vhost_toggle_device_iotlb(VirtIODevice *vdev)
> --
> 2.47.1
>