[PATCH] vhost: use try_lock in rte_vhost_vring_call

[Liu, Changpeng]

Hi Maxime,

> -----Original Message-----
> From: Maxime Coquelin <maxime.coquelin at redhat.com>
> Sent: Tuesday, September 20, 2022 3:19 PM
> To: Liu, Changpeng <changpeng.liu at intel.com>; dev at dpdk.org
> Cc: Xia, Chenbo <chenbo.xia at intel.com>
> Subject: Re: [PATCH] vhost: use try_lock in rte_vhost_vring_call
> 
> 
> 
> On 9/6/22 04:22, Changpeng Liu wrote:
> > Note that this function is in data path, so the thread context
> > may not same as socket messages processing context, by using
> > try_lock here, users can have another try in case of VQ's access
> > lock is held by `vhost-events` thread.
> >
> > Signed-off-by: Changpeng Liu <changpeng.liu at intel.com>
> > ---
> >   lib/vhost/vhost.c | 6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/lib/vhost/vhost.c b/lib/vhost/vhost.c
> > index 60cb05a0ff..072d2acb7b 100644
> > --- a/lib/vhost/vhost.c
> > +++ b/lib/vhost/vhost.c
> > @@ -1329,7 +1329,11 @@ rte_vhost_vring_call(int vid, uint16_t vring_idx)
> >   	if (!vq)
> >   		return -1;
> >
> > -	rte_spinlock_lock(&vq->access_lock);
> > +	if (!rte_spinlock_trylock(&vq->access_lock)) {
> > +		VHOST_LOG_CONFIG(dev->ifname, DEBUG,
> > +			"failed to kick guest, virtqueue busy.\n");
> > +		return -1;
> > +	}
> >
> >   	if (vq_is_packed(dev))
> >   		vhost_vring_call_packed(dev, vq);
> 
> I think that's problematic, because it will break other applications
> that currently rely on the API to block until the call is done.
> 
> Just some internal DPDK usage of this API:
> ./drivers/vdpa/ifc/ifcvf_vdpa.c:871:	rte_vhost_vring_call(internal->vid,
> qid);
> ./examples/vhost/virtio_net.c:236:	rte_vhost_vring_call(dev->vid, queue_id);
> ./examples/vhost/virtio_net.c:446:	rte_vhost_vring_call(dev->vid, queue_id);
> ./examples/vhost_blk/vhost_blk.c:99:
> rte_vhost_vring_call(task->ctrlr->vid, vq->id);
> ./examples/vhost_blk/vhost_blk.c:134:
> rte_vhost_vring_call(task->ctrlr->vid, vq->id);
> 
> This change will break all the above uses.
> 
> And that's not counting external projects.
> 
> ou should better introduce a new API that does not block.
Could you add a new API to do this? 
I think we can use the new API in SPDK as a workaround, note that SPDK project is blocked for
a while which can't be used with DPDK 22.05 or newer.
Vhost-blk and scsi devices are not same with vhost-net, we need to cover SeaBIOS and VM
cases, so we need to start processing vrings after 1 vring is ready.
> 
> Regards,
> Maxime