Re: [Qemu-devel] [PATCH v4] vhost: Don't abort when vhost-user connection is lost during migration

[Ying Fang]

On 2017/12/1 22:39, Michael S. Tsirkin wrote:
> On Fri, Dec 01, 2017 at 01:58:32PM +0800, fangying wrote:
>> QEMU will abort when vhost-user process is restarted during migration
>> when vhost_log_global_start/stop is called. The reason is clear that
>> vhost_dev_set_log returns -1 because network connection is lost.
>>
>> To handle this situation, let's cancel migration by setting migrate
>> state to failure and report it to user.
> 
> In fact I don't see this as the right way to fix it. Backend is dead so why
> not just proceed with migration? We just need to make sure we re-send
> migration data on re-connect.
> This is where vhost start/stop migration dirty log. The original code aborts
qemu here beacuse vhost data stream may break down if we fail to start/stop
vhost dirty log during migration. Backend may be active after 
vhost_log_global_start.

             dirty log start ----------------- dirty log stop
                     ^           ^
                     |           |
----- backend dead ----- backend active

Currently we don't re-send migration data on re-connect in this situation.
May we should work it out.

>> ---
>>  hw/virtio/vhost.c | 12 ++++++++++--
>>  1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>> index ddc42f0..92725f7 100644
>> --- a/hw/virtio/vhost.c
>> +++ b/hw/virtio/vhost.c
>> @@ -26,6 +26,8 @@
>>  #include "hw/virtio/virtio-bus.h"
>>  #include "hw/virtio/virtio-access.h"
>>  #include "migration/blocker.h"
>> +#include "migration/migration.h"
>> +#include "migration/qemu-file.h"
>>  #include "sysemu/dma.h"
>>  
>>  /* enabled until disconnected backend stabilizes */
>> @@ -885,7 +887,10 @@ static void vhost_log_global_start(MemoryListener 
>> *listener)
>>  
>>      r = vhost_migration_log(listener, true);
>>      if (r < 0) {
>> -        abort();
>> +        error_report("Failed to start vhost dirty log");
>> +        if (migrate_get_current()->migration_thread_running) {
>> +            qemu_file_set_error(migrate_get_current()->to_dst_file, 
>> -ECHILD);
>> +        }
>>      }
>>  }
>>  
>> @@ -895,7 +900,10 @@ static void vhost_log_global_stop(MemoryListener 
>> *listener)
>>  
>>      r = vhost_migration_log(listener, false);
>>      if (r < 0) {
>> -        abort();
>> +        error_report("Failed to stop vhost dirty log");
>> +        if (migrate_get_current()->migration_thread_running) {
>> +            qemu_file_set_error(migrate_get_current()->to_dst_file, 
>> -ECHILD);
>> +        }
>>      }
>>  }
>>  
>> -- 
>> 1.8.3.1
>>
> 
> .
>