Re: [Qemu-devel] [PATCH 1/2] libvhost-user: support resuming vq->last_avail_idx based on used_idx

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Aug 29, 2017 at 04:48:04PM +0200, Marc-André Lureau wrote:
> This is the same workaround as commit 523b018dde3b765, which was lost
> with libvhost-user transition in commit e10e798c85c2331.
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  contrib/libvhost-user/libvhost-user.h |  5 +++++
>  contrib/libvhost-user/libvhost-user.c | 13 +++++++++++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/contrib/libvhost-user/libvhost-user.h b/contrib/libvhost-user/libvhost-user.h
> index 53ef222c0b..aca71717c1 100644
> --- a/contrib/libvhost-user/libvhost-user.h
> +++ b/contrib/libvhost-user/libvhost-user.h
> @@ -132,6 +132,7 @@ typedef void (*vu_set_features_cb) (VuDev *dev, uint64_t features);
>  typedef int (*vu_process_msg_cb) (VuDev *dev, VhostUserMsg *vmsg,
>                                    int *do_reply);
>  typedef void (*vu_queue_set_started_cb) (VuDev *dev, int qidx, bool started);
> +typedef bool (*vu_queue_resume_to_used_cb) (VuDev *dev, int qidx);
>  
>  typedef struct VuDevIface {
>      /* called by VHOST_USER_GET_FEATURES to get the features bitmask */
> @@ -148,6 +149,10 @@ typedef struct VuDevIface {
>      vu_process_msg_cb process_msg;
>      /* tells when queues can be processed */
>      vu_queue_set_started_cb queue_set_started;
> +    /* If the queue must be resumed to vring.used->idx. */
> +    /* This can help to support resuming on unmanaged exit/crash when
> +     * the slave/driver processes elements in order. */
> +    vu_queue_resume_to_used_cb queue_resume_to_used;
>  } VuDevIface;
>  
>  typedef void (*vu_queue_handler_cb) (VuDev *dev, int qidx);

Let's consider: this is only safe if queue is processed in order.
So I think that would be a better name for the field.


> diff --git a/contrib/libvhost-user/libvhost-user.c b/contrib/libvhost-user/libvhost-user.c
> index 35fa0c5e56..36439caed6 100644
> --- a/contrib/libvhost-user/libvhost-user.c
> +++ b/contrib/libvhost-user/libvhost-user.c
> @@ -521,6 +521,19 @@ vu_set_vring_addr_exec(VuDev *dev, VhostUserMsg *vmsg)
>  
>      vq->used_idx = vq->vring.used->idx;
>  
> +    if (vq->last_avail_idx != vq->used_idx) {
> +        bool resume = dev->iface->queue_resume_to_used &&
> +            dev->iface->queue_resume_to_used(dev, index);
> +
> +        DPRINT("Last avail index != used index: %u != %u%s\n",
> +               vq->last_avail_idx, vq->used_idx,
> +               resume ? ", resuming" : "");
> +
> +        if (resume) {
> +            vq->shadow_avail_idx = vq->last_avail_idx = vq->used_idx;
> +        }
> +    }
> +
>      return false;
>  }
>  
> -- 
> 2.14.1.146.gd35faa819