Re: [Qemu-devel] [PATCH] scsi: esp: fix migration

[Paolo Bonzini]

On 27/06/2016 09:20, Amit Shah wrote:
> On (Mon) 20 Jun 2016 [16:33:26], Paolo Bonzini wrote:
>> Commit 926cde5 ("scsi: esp: make cmdbuf big enough for maximum CDB size",
>> 2016-06-16) changed the size of a migrated field.  Split it in two
>> parts, and only migrate the second part in a new vmstate version.
> 
> With this patch, the static checker fails in this way:
> 
> Section "esp", Description "esp": expected field "cmdlen", got "cmdbuf"; 
> skipping rest
> Section "dc390", Description "esp": expected field "cmdlen", got "cmdbuf"; 
> skipping rest
> Section "am53c974", Description "esp": expected field "cmdlen", got "cmdbuf"; 
> skipping rest
> 
> Note it doesn't complain about the version numbers.  That's because:
> 
>>  const VMStateDescription vmstate_esp = {
>>      .name ="esp",
>> -    .version_id = 3,
>> +    .version_id = 4,
>>      .minimum_version_id = 3,
> 
> this suggests older versions can still be accepted for incoming
> migration, which isn't true.

Sure they can:

-        VMSTATE_BUFFER(cmdbuf, ESPState),
+        VMSTATE_PARTIAL_BUFFER(cmdbuf, ESPState, 16),
+        VMSTATE_BUFFER_START_MIDDLE_V(cmdbuf, ESPState, 16, 4),

2.6 is transmitting version 3 and a 16-byte buffer.

2.7 is transmitting version 4, a first 16-byte buffer, and a second
16-byte buffer that is skipped when receiving version 3.

So it seems like a static checker limitation.

Paolo