Re: [PATCH v2 5/5] vhost: add device started check in migration set log

[Dima Stepanov]

On Thu, May 14, 2020 at 03:34:24PM +0800, Jason Wang wrote:
> 
> On 2020/5/13 下午5:47, Dima Stepanov wrote:
> >>>     case CHR_EVENT_CLOSED:
> >>>         /* a close event may happen during a read/write, but vhost
> >>>          * code assumes the vhost_dev remains setup, so delay the
> >>>          * stop & clear to idle.
> >>>          * FIXME: better handle failure in vhost code, remove bh
> >>>          */
> >>>         if (s->watch) {
> >>>             AioContext *ctx = qemu_get_current_aio_context();
> >>>
> >>>             g_source_remove(s->watch);
> >>>             s->watch = 0;
> >>>             qemu_chr_fe_set_handlers(&s->chr, NULL, NULL, NULL, NULL,
> >>>                                      NULL, NULL, false);
> >>>
> >>>             aio_bh_schedule_oneshot(ctx, chr_closed_bh, opaque);
> >>>         }
> >>>         break;
> >>>
> >>>I think it's time we dropped the FIXME and moved the handling to common
> >>>code. Jason? Marc-André?
> >>I agree. Just to confirm, do you prefer bh or doing changes like what is
> >>done in this series? It looks to me bh can have more easier codes.
> >Could it be a good idea just to make disconnect in the char device but
> >postphone clean up in the vhost-user-blk (or any other vhost-user
> >device) itself? So we are moving the postphone logic and decision from
> >the char device to vhost-user device. One of the idea i have is as
> >follows:
> >   - Put ourself in the INITIALIZATION state
> >   - Start these vhost-user "handshake" commands
> >   - If we got a disconnect error, perform disconnect, but don't clean up
> >     device (it will be clean up on the roll back). I can be done by
> >     checking the state in vhost_user_..._disconnect routine or smth like it
> 
> 
> Any issue you saw just using the aio bh as Michael posted above.
> 
> Then we don't need to deal with the silent vhost_dev_stop() and we will have
> codes that is much more easier to understand.
I've implemented this solution inside
hw/block/vhost-user-blk.c:vhost_user_blk_event() in the similar way by
using the s->connected field. Looks good and more correct fix ). I have
two questions here before i'll rework the fixes:
1. Is it okay to make the similar fix inside vhost_user_blk_event() or
we are looking for more generic vhost-user solution? What do you think?
2. For migration we require an additional information that for the
vhost-user device it isn't an error, because i'm trigerring the
following assert error:
  Core was generated by `x86_64-softmmu/qemu-system-x86_64 -nodefaults 
-no-user-config -M q35,sata=false'.
  Program terminated with signal SIGABRT, Aborted.
  #0  0x00007fb56e729428 in raise () from /lib/x86_64-linux-gnu/libc.so.6
  [Current thread is 1 (Thread 0x7fb486ef5700 (LWP 527734))]

  (gdb) bt
  #0  0x00007fb56e729428 in raise () from /lib/x86_64-linux-gnu/libc.so.6
  #1  0x00007fb56e72b02a in abort () from /lib/x86_64-linux-gnu/libc.so.6
  #2  0x00005648ea376ee6 in vhost_log_global_start
      (listener=0x5648ece4eb08) at ./hw/virtio/vhost.c:857
  #3  0x00005648ea2dde7e in memory_global_dirty_log_start ()
      at ./memory.c:2611
  #4  0x00005648ea2e68e7 in ram_init_bitmaps (rs=0x7fb4740008c0)
      at ./migration/ram.c:2305
  #5  0x00005648ea2e698b in ram_init_all (rsp=0x5648eb1f0f20 <ram_state>)
      at ./migration/ram.c:2323
  #6  0x00005648ea2e6cc5 in ram_save_setup (f=0x5648ec609e00,
      opaque=0x5648eb1f0f20 <ram_state>)
      at ./migration/ram.c:2436
  #7  0x00005648ea67b7d3 in qemu_savevm_state_setup (f=0x5648ec609e00) at
      migration/savevm.c:1176
  #8  0x00005648ea674511 in migration_thread (opaque=0x5648ec031ff0) at
      migration/migration.c:3416
  #9  0x00005648ea85d65d in qemu_thread_start (args=0x5648ec6057f0) at
      util/qemu-thread-posix.c:519
  #10 0x00007fb56eac56ba in start_thread () from
      /lib/x86_64-linux-gnu/libpthread.so.0
  #11 0x00007fb56e7fb41d in clone () from /lib/x86_64-linux-gnu/libc.so.6
  (gdb) frame 2
  #2  0x00005648ea376ee6 in vhost_log_global_start
     (listener=0x5648ece4eb08) at ./hw/virtio/vhost.c:857
  857             abort();
  (gdb) list
  852     {
  853         int r;
  854
  855         r = vhost_migration_log(listener, true);
  856         if (r < 0) {
  857             abort();
  858         }
  859     }
  860
  861     static void vhost_log_global_stop(MemoryListener *listener)
Since bh postphone the clean up, we can't use the ->started field.
Do we have any mechanism to get the device type/state in the common
vhost_migration_log() routine? So for example for the vhost-user/disconnect
device we will be able to return 0. Or should we implement it and introduce
it in this patch set?

Thanks, Dima.

> 
> Thank
> 
> 
> >   - vhost-user command returns error back to the _start() routine
> >   - Rollback in one place in the start() routine, by calling this
> >     postphoned clean up for the disconnect
> >
>