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

[Laurent Vivier <lvivier@redhat.com>]

On 20/01/2025 18:33, Eric Auger 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);
> +    }
>   }
>   
>   void vhost_toggle_device_iotlb(VirtIODevice *vdev)

I think you need the counterpart in vhost_iommu_region_del() (for instance if we have an 
add after a del that results in an empty list).
But you cannot unconditionally enable it (for instance if vhost is not started)

Perhaps you should move the vhost_set_iotlb_callback() call from 
vhost_start()/vhost_stop() to vhost_iommu_region_add()/vhost_iommu_region_del()?

Thanks,
Laurent