linux-kernel - Re: [PATCH 2/2] vringh: fetch used_idx from vring at vringh_init

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Date:   Wed, 1 Feb 2023 11:11:25 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Eugenio Pérez <eperezma@...hat.com>
Cc:     leiyang@...hat.com, Laurent Vivier <lvivier@...hat.com>,
        sgarzare@...hat.com, jasowang@...hat.com,
        Zhu Lingshan <lingshan.zhu@...el.com>,
        virtualization@...ts.linux-foundation.org, si-wei.liu@...cle.com,
        linux-kernel@...r.kernel.org, lulu@...hat.com,
        Gautam Dawar <gdawar@...inx.com>, alvaro.karsz@...id-run.com
Subject: Re: [PATCH 2/2] vringh: fetch used_idx from vring at
 vringh_init_iotlb

On Wed, Jan 18, 2023 at 05:43:59PM +0100, Eugenio Pérez wrote:
> Starting from an used_idx different than 0 is needed in use cases like
> virtual machine migration.  Not doing so and letting the caller set an
> avail idx different than 0 causes destination device to try to use old
> buffers that source driver already recover and are not available
> anymore.
> 
> While callers like vdpa_sim set avail_idx directly it does not set
> used_idx.  Instead of let the caller do the assignment, fetch it from
> the guest at initialization like vhost-kernel do.
> 
> To perform the same at vring_kernel_init and vring_user_init is left for
> the future.
> 
> Signed-off-by: Eugenio Pérez <eperezma@...hat.com>


So I applied 1/2 and dropped 2/2 for now, right?

> ---
>  drivers/vhost/vringh.c | 25 +++++++++++++++++++++++--
>  1 file changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> index 33eb941fcf15..0eed825197f2 100644
> --- a/drivers/vhost/vringh.c
> +++ b/drivers/vhost/vringh.c
> @@ -1301,6 +1301,17 @@ static inline int putused_iotlb(const struct vringh *vrh,
>  	return 0;
>  }
>  
> +/**
> + * vringh_update_used_idx - fetch used idx from driver's used split vring
> + * @vrh: The vring.
> + *
> + * Returns -errno or 0.
> + */
> +static inline int vringh_update_used_idx(struct vringh *vrh)
> +{
> +	return getu16_iotlb(vrh, &vrh->last_used_idx, &vrh->vring.used->idx);
> +}
> +
>  /**
>   * vringh_init_iotlb - initialize a vringh for a ring with IOTLB.
>   * @vrh: the vringh to initialize.
> @@ -1319,8 +1330,18 @@ int vringh_init_iotlb(struct vringh *vrh, u64 features,
>  		      struct vring_avail *avail,
>  		      struct vring_used *used)
>  {
> -	return vringh_init_kern(vrh, features, num, weak_barriers,
> -				desc, avail, used);
> +	int r = vringh_init_kern(vrh, features, num, weak_barriers, desc,
> +				 avail, used);
> +
> +	if (r != 0)
> +		return r;
> +
> +	/* Consider the ring not initialized */
> +	if ((void *)desc == used)
> +		return 0;
> +
> +	return vringh_update_used_idx(vrh);
> +
>  }
>  EXPORT_SYMBOL(vringh_init_iotlb);
>  
> -- 
> 2.31.1