Re: [PATCH] vhost-vdpa: filter VIRTIO_F_RING_PACKED feature

[Michael S. Tsirkin]

On Mon, Jun 05, 2023 at 01:06:44PM +0200, Stefano Garzarella wrote:
> vhost-vdpa IOCTLs (eg. VHOST_GET_VRING_BASE, VHOST_SET_VRING_BASE)
> don't support packed virtqueue well yet, so let's filter the
> VIRTIO_F_RING_PACKED feature for now in vhost_vdpa_get_features().
> 
> This way, even if the device supports it, we don't risk it being
> negotiated, then the VMM is unable to set the vring state properly.
> 
> Fixes: 4c8cf31885f6 ("vhost: introduce vDPA-based backend")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> ---
> 
> Notes:
>     This patch should be applied before the "[PATCH v2 0/3] vhost_vdpa:
>     better PACKED support" series [1] and backported in stable branches.
>     
>     We can revert it when we are sure that everything is working with
>     packed virtqueues.
>     
>     Thanks,
>     Stefano
>     
>     [1] https://lore.kernel.org/virtualization/20230424225031.18947-1-shannon.nelson@xxxxxxx/

I'm a bit lost here. So why am I merging "better PACKED support" then?
Does this patch make them a NOP?

>  drivers/vhost/vdpa.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> index 8c1aefc865f0..ac2152135b23 100644
> --- a/drivers/vhost/vdpa.c
> +++ b/drivers/vhost/vdpa.c
> @@ -397,6 +397,12 @@ static long vhost_vdpa_get_features(struct vhost_vdpa *v, u64 __user *featurep)
>  
>  	features = ops->get_device_features(vdpa);
>  
> +	/*
> +	 * IOCTLs (eg. VHOST_GET_VRING_BASE, VHOST_SET_VRING_BASE) don't support
> +	 * packed virtqueue well yet, so let's filter the feature for now.
> +	 */
> +	features &= ~BIT_ULL(VIRTIO_F_RING_PACKED);
> +
>  	if (copy_to_user(featurep, &features, sizeof(features)))
>  		return -EFAULT;
>  
> 
> base-commit: 9561de3a55bed6bdd44a12820ba81ec416e705a7
> -- 
> 2.40.1