Re: [PATCH RFC] virtio-fs: force virtio 1.x usage

[Michael S. Tsirkin]

On Mon, Jun 29, 2020 at 12:27:58PM +0200, Cornelia Huck wrote:
> virtio-fs devices are only specified for virtio-1, so it is unclear
> how a legacy or transitional device should behave.
> 
> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> ---
> 
> Forcing off legacy now (after the virtio-fs device has already been
> available) may have unintended consequences, therefore RFC.
> 
> By default, a virtio-pci device uses 'AUTO' for disable_legacy, which
> will resolve to different values based upon which bus the device has
> been plugged. Therefore, forcing disable_legacy may result in the same
> device or a quite different one.
> 
> Even though pre-virtio-1 behaviour of virtio-fs devices is simply not
> specified, toggling disable_legacy will have implications for the BAR
> layout, IIRC, and therefore a guest might end up getting a different
> device, even if it always used it with virtio-1 anyway.
> 
> Not sure what the best way to solve this problem is. Adding a compat
> property for disable_legacy=AUTO may be the right thing to do, but I'm
> not quite clear if there are any further implications here.

Well I notice that this device is not migrateable.
So I think that we can just switch it over and be done with it.


> Whatever we do here, we should make sure that the ccw incarnation of
> this device indeed forces virtio-1.

I agree. I notice that the API virtio_pci_force_virtio_1 turned out
to be too fragile. I propose that instead we have a whitelist of
devices which can be legacy or transitional. Force rest to modern.


> ---
>  hw/virtio/vhost-user-fs-pci.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/hw/virtio/vhost-user-fs-pci.c b/hw/virtio/vhost-user-fs-pci.c
> index e11c889d82b3..244205edf765 100644
> --- a/hw/virtio/vhost-user-fs-pci.c
> +++ b/hw/virtio/vhost-user-fs-pci.c
> @@ -44,6 +44,7 @@ static void vhost_user_fs_pci_realize(VirtIOPCIProxy 
> *vpci_dev, Error **errp)
>          vpci_dev->nvectors = dev->vdev.conf.num_request_queues + 2;
>      }
>  
> +    virtio_pci_force_virtio_1(vpci_dev);
>      qdev_realize(vdev, BUS(&vpci_dev->bus), errp);
>  }
>  
> -- 
> 2.25.4