Re: [PATCH v4 1/7] vhost: recheck dev state in the vhost_migration_log routine

[Dima Stepanov <dimastep@yandex-team.ru>]

On Tue, Sep 08, 2020 at 03:25:20PM +0100, Stefan Hajnoczi wrote:
> On Fri, Sep 04, 2020 at 12:31:13PM +0300, Dima Stepanov wrote:
> > vhost-user devices can get a disconnect in the middle of the VHOST-USER
> > handshake on the migration start. If disconnect event happened right
> > before sending next VHOST-USER command, then the vhost_dev_set_log()
> > call in the vhost_migration_log() function will return error. This error
> > will lead to the assert() and close the QEMU migration source process.
> > For the vhost-user devices the disconnect event should not break the
> > migration process, because:
> >   - the device will be in the stopped state, so it will not be changed
> >     during migration
> >   - if reconnect will be made the migration log will be reinitialized as
> >     part of reconnect/init process:
> >     #0  vhost_log_global_start (listener=0x563989cf7be0)
> >     at hw/virtio/vhost.c:920
> >     #1  0x000056398603d8bc in listener_add_address_space (listener=0x563989cf7be0,
> >         as=0x563986ea4340 <address_space_memory>)
> >     at softmmu/memory.c:2664
> >     #2  0x000056398603dd30 in memory_listener_register (listener=0x563989cf7be0,
> >         as=0x563986ea4340 <address_space_memory>)
> >     at softmmu/memory.c:2740
> >     #3  0x0000563985fd6956 in vhost_dev_init (hdev=0x563989cf7bd8,
> >         opaque=0x563989cf7e30, backend_type=VHOST_BACKEND_TYPE_USER,
> >         busyloop_timeout=0)
> >     at hw/virtio/vhost.c:1385
> >     #4  0x0000563985f7d0b8 in vhost_user_blk_connect (dev=0x563989cf7990)
> >     at hw/block/vhost-user-blk.c:315
> >     #5  0x0000563985f7d3f6 in vhost_user_blk_event (opaque=0x563989cf7990,
> >         event=CHR_EVENT_OPENED)
> >     at hw/block/vhost-user-blk.c:379
> > Update the vhost-user-blk device with the internal started_vu field which
> > will be used for initialization (vhost_user_blk_start) and clean up
> > (vhost_user_blk_stop). This additional flag in the VhostUserBlk structure
> > will be used to track whether the device really needs to be stopped and
> > cleaned up on a vhost-user level.
> > The disconnect event will set the overall VHOST device (not vhost-user) to
> > the stopped state, so it can be used by the general vhost_migration_log
> > routine.
> > Such approach could be propogated to the other vhost-user devices, but
> > better idea is just to make the same connect/disconnect code for all the
> > vhost-user devices.
> > 
> > This migration issue was slightly discussed earlier:
> >   - https://lists.gnu.org/archive/html/qemu-devel/2020-05/msg01509.html
> >   - https://lists.gnu.org/archive/html/qemu-devel/2020-05/msg05241.html
> > 
> > Signed-off-by: Dima Stepanov <dimastep@yandex-team.ru>
> > Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
> > ---
> >  hw/block/vhost-user-blk.c          | 19 ++++++++++++++++---
> >  hw/virtio/vhost.c                  | 27 ++++++++++++++++++++++++---
> >  include/hw/virtio/vhost-user-blk.h | 10 ++++++++++
> >  3 files changed, 50 insertions(+), 6 deletions(-)
> > 
> > diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> > index 39aec42..a076b1e 100644
> > --- a/hw/block/vhost-user-blk.c
> > +++ b/hw/block/vhost-user-blk.c
> > @@ -150,6 +150,7 @@ static int vhost_user_blk_start(VirtIODevice *vdev)
> >          error_report("Error starting vhost: %d", -ret);
> >          goto err_guest_notifiers;
> >      }
> > +    s->started_vu = true;
> >  
> >      /* guest_notifier_mask/pending not used yet, so just unmask
> >       * everything here. virtio-pci will do the right thing by
> > @@ -175,6 +176,11 @@ static void vhost_user_blk_stop(VirtIODevice *vdev)
> >      VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
> >      int ret;
> >  
> > +    if (!s->started_vu) {
> > +        return;
> > +    }
> > +    s->started_vu = false;
> > +
> >      if (!k->set_guest_notifiers) {
> >          return;
> >      }
> > @@ -341,9 +347,7 @@ static void vhost_user_blk_disconnect(DeviceState *dev)
> >      }
> >      s->connected = false;
> >  
> > -    if (s->dev.started) {
> > -        vhost_user_blk_stop(vdev);
> > -    }
> > +    vhost_user_blk_stop(vdev);
> >  
> >      vhost_dev_cleanup(&s->dev);
> >  }
> > @@ -399,6 +403,15 @@ static void vhost_user_blk_event(void *opaque, QEMUChrEvent event)
> >                      NULL, NULL, false);
> >              aio_bh_schedule_oneshot(ctx, vhost_user_blk_chr_closed_bh, opaque);
> >          }
> > +
> > +        /*
> > +         * Move vhost device to the stopped state. The vhost-user device
> > +         * will be clean up and disconnected in BH. This can be useful in
> > +         * the vhost migration code. If disconnect was caught there is an
> > +         * option for the general vhost code to get the dev state without
> > +         * knowing its type (in this case vhost-user).
> > +         */
> > +        s->dev.started = false;
> >          break;
> >      case CHR_EVENT_BREAK:
> >      case CHR_EVENT_MUX_IN:
> 
> Hi Dima,
> Is it possible to move this logic into the vhost_*() API so that all
> devices benefit from it? This seems like a generic vhost-user issue
> rather than a vhost-user-blk issue.
> 
> In other words, it would be great if the vhost APIs in QEMU are designed
> in a way so that the user doesn't need to think about this.
> 
> Stefan

Hi Stefan,

Yes, i also think that it is a right direction, to have some
vhost/vhost-user API here. In this case we will be able to share the
code between all the vhost-user devices, which is important.

We had the same conclusion in this e-mail thread:
https://lists.gnu.org/archive/html/qemu-devel/2020-05/msg06955.html
When we backported the fix from the vhost-user-net device to
vhost-user-blk and also provided some additional logic only for the
vhost-user-blk device.
So my idea is, first fix two issues:
- vhost-user-blk reconnect issue (already merged)
- vhost-user-blk migration issue (this patchset)
After it try to review current implementation, so for example the unix
socket logic could be shared between all the vhost-user devices. Maybe
something else could be done here.

Dima.