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

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

On Mon, May 18, 2020 at 10:52:08AM +0800, Jason Wang wrote:
> 
> On 2020/5/16 上午11:20, Li Feng wrote:
> >Hi, Dima.
> >This abort is what I have mentioned in my previous email.
> >I have triggered this crash without any fix a week ago.
> >And I have written a test patch to let vhost_log_global_start return
> >int and propagate the error to up layer.
> >However, my change is a little large, because the origin callback
> >return void, and don't do some rollback.
> >After test, the migration could migrate to dst successfully, and fio
> >is still running perfectly, but the src vm is still stuck here, no
> >crash.
> >
> >Is it right to return this error to the up layer?
> 
> 
> That could be a solution or we may ask David for more suggestion.
> 
> Another thing that might be useful is to block re connection during
> migration.
I've written a little more information as answer to Feng's mail. But
what if add some new callback to get the device started state (started or not).
And for the vhost-user (or at least vhost-usr-blk) devices it will use
the connected field also to return the device state:
  - disconnect -> not started
For other devices we can just return the started field value as it is
right now.

No other comments mixed in below.

> 
> Thanks
> 
> 
> >
> >Thanks,
> >Feng Li
> >
> >Dima Stepanov <dimastep@yandex-team.ru> 于2020年5月16日周六 上午12:55写道：
> >>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
> >>>>
>