Subject: Re: [PATCH] vdpa: reject F_ENABLE_AFTER_DRIVER_OK if backend does not support it

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 3 Jul 2023 10:52:03 -0400

On Mon, Jul 03, 2023 at 04:22:18PM +0200, Eugenio Pérez wrote:
> With the current code it is accepted as long as userland send it.
> 
> Although userland should not set a feature flag that has not been
> offered to it with VHOST_GET_BACKEND_FEATURES, the current code will not
> complain for it.
> 
> Since there is no specific reason for any parent to reject that backend
> feature bit when it has been proposed, let's control it at vdpa frontend
> level. Future patches may move this control to the parent driver.
> 
> Fixes: 967800d2d52e ("vdpa: accept VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK backend feature")
> Signed-off-by: Eugenio Pérez <[email-masked]>

Please do send v3. And again, I don't want to send "after driver ok" hack
upstream at all, I merged it in next just to give it some testing.
We want RING_ACCESS_AFTER_KICK or some such.

> ---
> Sent with Fixes: tag pointing to git.kernel.org/pub/scm/linux/kernel/git/mst
> commit. Please let me know if I should send a v3 of [1] instead.
> 
> [1] https://lore.kernel.org/lkml/[email-masked]/T/
> ---
>  drivers/vhost/vdpa.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> index e1abf29fed5b..a7e554352351 100644
> --- a/drivers/vhost/vdpa.c
> +++ b/drivers/vhost/vdpa.c
> @@ -681,18 +681,21 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
>  {
>  	struct vhost_vdpa *v = filep->private_data;
>  	struct vhost_dev *d = &v->vdev;
> +	const struct vdpa_config_ops *ops = v->vdpa->config;
>  	void __user *argp = (void __user *)arg;
>  	u64 __user *featurep = argp;
> -	u64 features;
> +	u64 features, parent_features = 0;
>  	long r = 0;
>  
>  	if (cmd == VHOST_SET_BACKEND_FEATURES) {
>  		if (copy_from_user(&features, featurep, sizeof(features)))
>  			return -EFAULT;
> +		if (ops->get_backend_features)
> +			parent_features = ops->get_backend_features(v->vdpa);
>  		if (features & ~(VHOST_VDPA_BACKEND_FEATURES |
>  				 BIT_ULL(VHOST_BACKEND_F_SUSPEND) |
>  				 BIT_ULL(VHOST_BACKEND_F_RESUME) |
> -				 BIT_ULL(VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK)))
> +				 parent_features))
>  			return -EOPNOTSUPP;
>  		if ((features & BIT_ULL(VHOST_BACKEND_F_SUSPEND)) &&
>  		     !vhost_vdpa_can_suspend(v))
> -- 
> 2.39.3