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

[Amit Shah]

On (Mon) 27 Jun 2016 [09:54:24], Paolo Bonzini wrote:
> 
> 
> 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.

Hm, yeah.

> So it seems like a static checker limitation.

Yeah, so I think this can't be checked statically -- let me dig more.


                Amit