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

[Michael S. Tsirkin]

On Wed, Mar 13, 2019 at 10:05:51AM +0800, Yongji Xie wrote:
> On Wed, 13 Mar 2019 at 00:49, Michael S. Tsirkin <address@hidden> 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(-)
> > >
> > > diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> > > index 9682df1a7b..539ea2e571 100644
> > > --- a/hw/block/vhost-user-blk.c
> > > +++ b/hw/block/vhost-user-blk.c
> > > @@ -103,7 +103,7 @@ const VhostDevConfigOps blk_ops = {
> > >      .vhost_dev_config_notifier = vhost_user_blk_handle_config_change,
> > >  };
> > >
> > > -static void vhost_user_blk_start(VirtIODevice *vdev)
> > > +static int vhost_user_blk_start(VirtIODevice *vdev)
> > >  {
> > >      VHostUserBlk *s = VHOST_USER_BLK(vdev);
> > >      BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(vdev)));
> > > @@ -112,13 +112,13 @@ static void vhost_user_blk_start(VirtIODevice *vdev)
> > >
> > >      if (!k->set_guest_notifiers) {
> > >          error_report("binding does not support guest notifiers");
> > > -        return;
> > > +        return -ENOSYS;
> > >      }
> > >
> > >      ret = vhost_dev_enable_notifiers(&s->dev, vdev);
> > >      if (ret < 0) {
> > >          error_report("Error enabling host notifiers: %d", -ret);
> > > -        return;
> > > +        return ret;
> > >      }
> > >
> > >      ret = k->set_guest_notifiers(qbus->parent, s->dev.nvqs, true);
> > > @@ -157,12 +157,13 @@ static void vhost_user_blk_start(VirtIODevice *vdev)
> > >          vhost_virtqueue_mask(&s->dev, vdev, i, false);
> > >      }
> > >
> > > -    return;
> > > +    return ret;
> > >
> > >  err_guest_notifiers:
> > >      k->set_guest_notifiers(qbus->parent, s->dev.nvqs, false);
> > >  err_host_notifiers:
> > >      vhost_dev_disable_notifiers(&s->dev, vdev);
> > > +    return ret;
> > >  }
> > >
> > >  static void vhost_user_blk_stop(VirtIODevice *vdev)
> > > @@ -181,7 +182,6 @@ static void vhost_user_blk_stop(VirtIODevice *vdev)
> > >      ret = k->set_guest_notifiers(qbus->parent, s->dev.nvqs, false);
> > >      if (ret < 0) {
> > >          error_report("vhost guest notifier cleanup failed: %d", ret);
> > > -        return;
> > >      }
> > >
> > >      vhost_dev_disable_notifiers(&s->dev, vdev);
> > > @@ -191,21 +191,43 @@ static void vhost_user_blk_set_status(VirtIODevice 
> > > *vdev, uint8_t status)
> > >  {
> > >      VHostUserBlk *s = VHOST_USER_BLK(vdev);
> > >      bool should_start = status & VIRTIO_CONFIG_S_DRIVER_OK;
> > > +    int ret;
> > >
> > >      if (!vdev->vm_running) {
> > >          should_start = false;
> > >      }
> > >
> > > -    if (s->dev.started == should_start) {
> > > +    if (s->should_start == should_start) {
> > > +        return;
> > > +    }
> > > +
> > > +    if (!s->connected || s->dev.started == should_start) {
> > > +        s->should_start = should_start;
> > >          return;
> > >      }
> > >
> > >      if (should_start) {
> > > -        vhost_user_blk_start(vdev);
> > > +        s->should_start = true;
> > > +        /*
> > > +         * make sure vhost_user_blk_handle_output() ignores fake
> > > +         * guest kick by vhost_dev_enable_notifiers()
> > > +         */
> > > +        barrier();
> > > +        ret = vhost_user_blk_start(vdev);
> > > +        if (ret < 0) {
> > > +            error_report("vhost-user-blk: vhost start failed: %s",
> > > +                         strerror(-ret));
> > > +            qemu_chr_fe_disconnect(&s->chardev);
> > > +        }
> > >      } else {
> > >          vhost_user_blk_stop(vdev);
> > > +        /*
> > > +         * make sure vhost_user_blk_handle_output() ignore fake
> > > +         * guest kick by vhost_dev_disable_notifiers()
> > > +         */
> > > +        barrier();
> > > +        s->should_start = false;
> > >      }
> > > -
> > >  }
> > >
> > >  static uint64_t vhost_user_blk_get_features(VirtIODevice *vdev,
> > > @@ -237,13 +259,22 @@ static uint64_t 
> > > vhost_user_blk_get_features(VirtIODevice *vdev,
> > >  static void vhost_user_blk_handle_output(VirtIODevice *vdev, VirtQueue 
> > > *vq)
> > >  {
> > >      VHostUserBlk *s = VHOST_USER_BLK(vdev);
> > > -    int i;
> > > +    int i, ret;
> > >
> > >      if (!(virtio_host_has_feature(vdev, VIRTIO_F_VERSION_1) &&
> > >          !virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1))) {
> > >          return;
> > >      }
> > >
> > > +    if (s->should_start) {
> > > +        return;
> > > +    }
> > > +    s->should_start = true;
> > > +
> > > +    if (!s->connected) {
> > > +        return;
> > > +    }
> > > +
> > >      if (s->dev.started) {
> > >          return;
> > >      }
> > > @@ -251,7 +282,13 @@ static void 
> > > vhost_user_blk_handle_output(VirtIODevice *vdev, VirtQueue *vq)
> > >      /* Some guests kick before setting VIRTIO_CONFIG_S_DRIVER_OK so start
> > >       * vhost here instead of waiting for .set_status().
> > >       */
> > > -    vhost_user_blk_start(vdev);
> > > +    ret = vhost_user_blk_start(vdev);
> > > +    if (ret < 0) {
> > > +        error_report("vhost-user-blk: vhost start failed: %s",
> > > +                     strerror(-ret));
> > > +        qemu_chr_fe_disconnect(&s->chardev);
> > > +        return;
> > > +    }
> > >
> > >      /* Kick right away to begin processing requests already in vring */
> > >      for (i = 0; i < s->dev.nvqs; i++) {
> > > @@ -271,13 +308,106 @@ static void vhost_user_blk_reset(VirtIODevice 
> > > *vdev)
> > >      vhost_dev_free_inflight(s->inflight);
> > >  }
> > >
> > > +static int vhost_user_blk_connect(DeviceState *dev)
> > > +{
> > > +    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > +    VHostUserBlk *s = VHOST_USER_BLK(vdev);
> > > +    int ret = 0;
> > > +
> > > +    if (s->connected) {
> > > +        return 0;
> > > +    }
> > > +    s->connected = true;
> > > +
> > > +    s->dev.nvqs = s->num_queues;
> > > +    s->dev.vqs = s->vqs;
> > > +    s->dev.vq_index = 0;
> > > +    s->dev.backend_features = 0;
> > > +
> > > +    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_report("vhost-user-blk: vhost initialization failed: %s",
> > > +                     strerror(-ret));
> > > +        return ret;
> > > +    }
> > > +
> > > +    /* restore vhost state */
> > > +    if (s->should_start) {
> > > +        ret = vhost_user_blk_start(vdev);
> > > +        if (ret < 0) {
> > > +            error_report("vhost-user-blk: vhost start failed: %s",
> > > +                         strerror(-ret));
> > > +            return ret;
> > > +        }
> > > +    }
> > > +
> > > +    return 0;
> > > +}
> > > +
> > > +static void vhost_user_blk_disconnect(DeviceState *dev)
> > > +{
> > > +    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > +    VHostUserBlk *s = VHOST_USER_BLK(vdev);
> > > +
> > > +    if (!s->connected) {
> > > +        return;
> > > +    }
> > > +    s->connected = false;
> > > +
> > > +    if (s->dev.started) {
> > > +        vhost_user_blk_stop(vdev);
> > > +    }
> > > +
> > > +    vhost_dev_cleanup(&s->dev);
> > > +}
> > > +
> > > +static gboolean vhost_user_blk_watch(GIOChannel *chan, GIOCondition cond,
> > > +                                     void *opaque)
> > > +{
> > > +    DeviceState *dev = opaque;
> > > +    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > +    VHostUserBlk *s = VHOST_USER_BLK(vdev);
> > > +
> > > +    qemu_chr_fe_disconnect(&s->chardev);
> > > +
> > > +    return true;
> > > +}
> > > +
> > > +static void vhost_user_blk_event(void *opaque, int event)
> > > +{
> > > +    DeviceState *dev = opaque;
> > > +    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > +    VHostUserBlk *s = VHOST_USER_BLK(vdev);
> > > +
> > > +    switch (event) {
> > > +    case CHR_EVENT_OPENED:
> > > +        if (vhost_user_blk_connect(dev) < 0) {
> > > +            qemu_chr_fe_disconnect(&s->chardev);
> > > +            return;
> > > +        }
> > > +        s->watch = qemu_chr_fe_add_watch(&s->chardev, G_IO_HUP,
> > > +                                         vhost_user_blk_watch, dev);
> > > +        break;
> > > +    case CHR_EVENT_CLOSED:
> > > +        vhost_user_blk_disconnect(dev);
> > > +        if (s->watch) {
> > > +            g_source_remove(s->watch);
> > > +            s->watch = 0;
> > > +        }
> > > +        break;
> > > +    }
> > > +}
> > > +
> > > +
> > >  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?
> >
> 
> Qemu is client in current vhost-user-blk implenment. Seems like
> backend will not try to connect.

Or just fail realize and make management retry?

> > 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.
> >
> 
> Yes, because I want to support the case that backend starts after
> qemu. I can print some warning and add timeout on this.
> 
> Thanks,
> Yongji

Well a chardev already has a reconnect option.
Can you use that machinery?
IIUC it works without blocking QEMU.

-- 
MST