Re: [PATCH 4/5] virtio: Fail if iommu_platform is requested, but unsupported

[Raphael Norwitz]

On Thu, Apr 22, 2021 at 07:02:20PM +0200, Kevin Wolf wrote:
> Commit 2943b53f6 (' virtio: force VIRTIO_F_IOMMU_PLATFORM') made sure
> that vhost can't just reject VIRTIO_F_IOMMU_PLATFORM when it was
> requested. However, just adding it back to the negotiated flags isn't
> right either because it promises support to the guest that the device
> actually doesn't support. One example of a vhost-user device that
> doesn't have support for the flag is the vhost-user-blk export of QEMU.
> 
> Instead of successfully creating a device that doesn't work, just fail
> to plug the device when it doesn't support the feature, but it was
> requested. This results in much clearer error messages.
> 
> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1935031
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  hw/virtio/virtio-bus.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
> index d6332d45c3..859978d248 100644
> --- a/hw/virtio/virtio-bus.c
> +++ b/hw/virtio/virtio-bus.c
> @@ -69,6 +69,11 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error 
> **errp)
>          return;
>      }
>

Can you explain this check a little more?

Above we have:
bool has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);

and then we get the host features from the bckend:
vdev->host_features = vdc->get_features(vdev, vdev->host_features

So as is this is catching the case where vdev->host_features had
VIRTIO_F_IOMMU_PLATFORM set before (by default?), but doesn't now that
the features have been retrieved? 

Why not just:
    if (!virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {

> +    if (has_iommu && !virtio_host_has_feature(vdev, 
> VIRTIO_F_IOMMU_PLATFORM)) {
> +        error_setg(errp, "iommu_platform=true is not supported by the 
> device");
> +        return;
> +    }
> +
>      if (klass->device_plugged != NULL) {
>          klass->device_plugged(qbus->parent, &local_err);
>      }
> -- 
> 2.30.2
>