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 05:18:11PM +0200, Marc-André Lureau wrote:
> Hi
> 
> On Tue, Aug 29, 2017 at 5:09 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Tue, Aug 29, 2017 at 05:04:34PM +0200, Marc-André Lureau wrote:
> >> Hi
> >>
> >> On Tue, Aug 29, 2017 at 5:01 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> >> > 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.
> >> >
> >> That's what the comment say already :)
> >>
> >> Which name do you suggest? vu_queue_is_processed_in_order?
> >
> > Something like this.
> >
> >> Does that
> >> imply it should always resume?
> >
> > Not sure I understand the question.
> 
> If a "queue_is_processed_in_order", does that imply libvhost-user
> should always resume it based on used_idx? I think it does, but the
> correlation is less obvious than explicitly saying "resume_to_used".

Just add a comment then. You do not want low level details
like used index in all backends.


> 
> -- 
> Marc-André Lureau