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

[Jason Wang]

On 2020/5/16 上午12:54, Dima Stepanov wrote:
> 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?


I think I agree with Michael, it's better to have a generic vhost-user 
solution. But if it turns out to be not easy, we can start from fixing 
vhost-user-blk.


> 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?


This requires more thought, I will reply in Feng's mail.

Thanks


> 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