* Dima Stepanov (dimas...@yandex-team.ru) wrote:
> On Mon, May 18, 2020 at 10:53:59AM +0100, Dr. David Alan Gilbert wrote:
> > * Dima Stepanov (dimas...@yandex-team.ru) wrote:
> > > On Mon, May 18, 2020 at 10:50:39AM +0800, Jason Wang wrote:
> > > > 
> > > > 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.
> > > I also agree, but as i see it right now the connect/disconnect events
> > > are handled inside each vhost-user device implementation file. So it will
> > > need some global refactoring. So i suggest having this fix first and
> > > after it refactoring the code:
> > >  - more devices will be involved
> > >  - i see there is some difference in device handling
> > 
> > I'm following bits of this discussion, some thoughts;
> > if your device doesn't support reconnect, then if, at the start of
> > migration you find that you can't start the log what is the correct
> > behaviour?
> I'm not sure here, but it looks like that in this case the device state
> will be:
>   disconnect -> stopped (will not be changed during migration, because
>   reconnect isn't supported)
> And because of it the device state will not be changed during migration,
> so there is no need for log and migration could be completed
> successfully.
> So as i see it (i could be wrong here) that:
>  - it is okay: if device is not started and we will not change this
>    state during migration + log start is failed
>  - it is not okay: if device is started + log start is failed (because
>    we can't handle the dirty pages and so on during migration)

Yes, that does make sense to me.

> > You can't carry on with the migration because you'd have an
> > inconsistent migration state; so I guess that's why the abort() is there
> > - but I think I'd generally prefer to fail the migration and hope the
> > vhsot device is still working for anything other than the log.
> > 
> > You're going to have to be pretty careful with the ordering of reconect
> > - reconnecting on the source during a migration sounds pretty hairy, but
> > a migration can take many minutes, so if you really want to survive this
> > I guess you have to.
> Maybe if we get a disconnect during migration then we could postphone or
> just don't make reconnect at all till the end of migration on the source
> side. This will make a device left in the stopped state.

Yes, that's the easiest way, but you may find people object to it being
out for that long.

Dave

> Thanks, Dima.
> 
> > 
> > Dave
> > 
> > 
> > > > 
> > > > 
> > > > >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.
> > > Okay, let's continue discussion there.
> > > 
> > > No other comments mixed in below.
> > > 
> > > Thanks, Dima.
> > > 
> > > > 
> > > > 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
> > > > >>>
> > > > 
> > > 
> > --
> > Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
> > 
> 
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK


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