Re: [PATCH 2/2] vringh: fetch used_idx from vring at vringh_init_iotlb

[Jason Wang]

On Thu, Jan 19, 2023 at 12:44 AM Eugenio Pérez <eperezma@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
>  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)
>  {

While at this, I wonder if it's better to have a dedicated parameter
for last_avail_idx?

> -       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;

I don't understand when we can get this (actually it should be a bug
of the caller).

Thanks

> +
> +       return vringh_update_used_idx(vrh);
> +
>  }
>  EXPORT_SYMBOL(vringh_init_iotlb);
>
> --
> 2.31.1
>