Re: [Qemu-devel] [PATCH v7 6/7] vhost-user-blk: Add support to reconnect backend

[Daniel P . Berrangé]

On Thu, Mar 14, 2019 at 03:31:47PM +0300, Yury Kotov wrote:
> Hi,
> 
> 14.03.2019, 14:44, "Daniel P. Berrangé" <address@hidden>:
> > On Thu, Mar 14, 2019 at 07:34:03AM -0400, Michael S. Tsirkin wrote:
> >>  On Thu, Mar 14, 2019 at 11:24:22AM +0000, Daniel P. Berrangé wrote:
> >>  > On Tue, Mar 12, 2019 at 12:49:35PM -0400, Michael S. Tsirkin wrote:
> >>  > > On Thu, Feb 28, 2019 at 04:53:54PM +0800, address@hidden wrote:
> >>  > > > From: Xie Yongji <address@hidden>
> >>  > > >
> >>  > > > Since we now support the message VHOST_USER_GET_INFLIGHT_FD
> >>  > > > and VHOST_USER_SET_INFLIGHT_FD. The backend is able to restart
> >>  > > > safely because it can track inflight I/O in shared memory.
> >>  > > > This patch allows qemu to reconnect the backend after
> >>  > > > connection closed.
> >>  > > >
> >>  > > > Signed-off-by: Xie Yongji <address@hidden>
> >>  > > > Signed-off-by: Ni Xun <address@hidden>
> >>  > > > Signed-off-by: Zhang Yu <address@hidden>
> >>  > > > ---
> >>  > > > hw/block/vhost-user-blk.c | 205 +++++++++++++++++++++++------
> >>  > > > include/hw/virtio/vhost-user-blk.h | 4 +
> >>  > > > 2 files changed, 167 insertions(+), 42 deletions(-)
> >>  >
> >>  >
> >>  > > > static void vhost_user_blk_device_realize(DeviceState *dev, Error 
> >> **errp)
> >>  > > > {
> >>  > > > VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> >>  > > > VHostUserBlk *s = VHOST_USER_BLK(vdev);
> >>  > > > VhostUserState *user;
> >>  > > > - struct vhost_virtqueue *vqs = NULL;
> >>  > > > int i, ret;
> >>  > > > + Error *err = NULL;
> >>  > > >
> >>  > > > if (!s->chardev.chr) {
> >>  > > > error_setg(errp, "vhost-user-blk: chardev is mandatory");
> >>  > > > @@ -312,27 +442,28 @@ static void 
> >> vhost_user_blk_device_realize(DeviceState *dev, Error **errp)
> >>  > > > }
> >>  > > >
> >>  > > > s->inflight = g_new0(struct vhost_inflight, 1);
> >>  > > > -
> >>  > > > - s->dev.nvqs = s->num_queues;
> >>  > > > - s->dev.vqs = g_new(struct vhost_virtqueue, s->dev.nvqs);
> >>  > > > - s->dev.vq_index = 0;
> >>  > > > - s->dev.backend_features = 0;
> >>  > > > - vqs = s->dev.vqs;
> >>  > > > -
> >>  > > > - vhost_dev_set_config_notifier(&s->dev, &blk_ops);
> >>  > > > -
> >>  > > > - ret = vhost_dev_init(&s->dev, s->vhost_user, 
> >> VHOST_BACKEND_TYPE_USER, 0);
> >>  > > > - if (ret < 0) {
> >>  > > > - error_setg(errp, "vhost-user-blk: vhost initialization failed: 
> >> %s",
> >>  > > > - strerror(-ret));
> >>  > > > - goto virtio_err;
> >>  > > > - }
> >>  > > > + s->vqs = g_new(struct vhost_virtqueue, s->num_queues);
> >>  > > > + s->watch = 0;
> >>  > > > + s->should_start = false;
> >>  > > > + s->connected = false;
> >>  > > > +
> >>  > > > + qemu_chr_fe_set_handlers(&s->chardev, NULL, NULL, 
> >> vhost_user_blk_event,
> >>  > > > + NULL, (void *)dev, NULL, true);
> >>  > > > +
> >>  > > > +reconnect:
> >>  > > > + do {
> >>  > > > + if (qemu_chr_fe_wait_connected(&s->chardev, &err) < 0) {
> >>  > > > + error_report_err(err);
> >>  > > > + err = NULL;
> >>  > > > + sleep(1);
> >>  > >
> >>  > > Seems arbitrary. Is this basically waiting until backend will 
> >> reconnect?
> >>  > > Why not block until event on the fd triggers?
> >>  > >
> >>  > > Also, it looks like this will just block forever with no monitor input
> >>  > > and no way for user to figure out what is going on short of
> >>  > > crashing QEMU.
> >>  >
> >>  > FWIW, the current vhost-user-net device does exactly the same thing
> >>  > with calling qemu_chr_fe_wait_connected during its realize() function.
> >>
> >>  Hmm yes. It doesn't sleep for an arbitrary 1 sec so less of an eyesore :)
> >
> > The sleep(1) in this patch simply needs to be removed. I think that
> > probably dates from when it was written against the earlier broken
> > version of qemu_chr_fe_wait_connected(). That would not correctly
> > deal with the "reconnect" flag, and so needing this loop with a sleep
> > in it.
> >
> > In fact the while loop can be removed as well in this code. It just
> > needs to call qemu_chr_fe_wait_connected() once. It is guaranteed
> > to have a connected peer once that returns 0.
> >
> > qemu_chr_fe_wait_connected() only returns -1 if the operating in
> > client mode, and it failed to connect and reconnect is *not*
> > requested. In such case the caller should honour the failure and
> > quit, not loop to retry.
> >
> > The reason vhost-user-net does a loop is because once it has a
> > connection it tries todo a protocol handshake, and if that
> > handshake fails it closes the chardev and tries to connect
> > again. That's not the case in this blk code os the loop is
> > not needed.
> >
> 
> But vhost-user-blk also has a handshake in device realize. What happens if the
> connection is broken during realization? IIUC we have to retry a handshake in
> such case just like vhost-user-net.

I'm just commenting on the current code which does not do that
handshake in the loop afaict. If it needs to do that then the
patch should be updated...


Regards,
Daniel
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