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

[Dima Stepanov]

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

> 
> 
> >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
> >>>
>