Hi,
14.03.2019, 14:44, "Daniel P. Berrangé" <berra...@redhat.com>:
> 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, elohi...@gmail.com wrote:
>> > > > From: Xie Yongji <xieyon...@baidu.com>
>> > > >
>> > > > 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 <xieyon...@baidu.com>
>> > > > Signed-off-by: Ni Xun <ni...@baidu.com>
>> > > > Signed-off-by: Zhang Yu <zhangy...@baidu.com>
>> > > > ---
>> > > > 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.
>> > Can't say I'm thrilled about the existing usage either, but I don't
>> > see a particularly nice alternative if it isn't possible to realize
>> > the device without having a backend available.
>> >
>> > I don't think this an especially serious problem during cold startup
>> > though since doesn't expect the monitor to become responsive until
>> > the device model is fully realized & all backends connected.
>> >
>> > The real concern is if you need to hotplug this kind of device
>> > at runtime. In that case blocking the main loop / monitor is a
>> > serious problem that will actively harm both the guest OS and
>> > mgmt applications.
>> >
>> > The vhost-user-net device will already suffer from that.
>>
>> Right.
>>
>> > To solve the problem with hotplug, the mgmt app would need to
>> > plug the chardev backend, then wait for it to become connected,
>> > and only then plug the device frontend. In that case we would not
>> > want this loop. We'd expect it to use the already connected
>> > chardev, and fail the realize() function if the chardev has
>> > lost its connection.
>>
>> I think as step one we should maybe just busy-wait with no sleep(1).
>> But respecting reconnect would be nice too.
>
> As above I don't see a need to loop at all.
>
>> And I think if we can't respect nowait we should
>> detect it and fail, in both net and block ...
>
> If the chardev is in server mode with "wait" set, or in client mode
> with reconnect not set, then there's actually no need to call
> qemu_chr_fe_wait_connected at all. The qemu_chr_fe_wait_connected is
> only needed if the chardev is a server with "nowait" set or a client
> with "reconnect" set, or if the caller explicitly closes the existing
> connection & wants to establish a new one.
>
> Regards,
> Daniel
> --
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Regards,
Yury
