Re: [Qemu-devel] [PATCH 3/4] virtio: modify savevm to have a stable wire format

[Anthony Liguori <anthony@codemonkey.ws>]

Rusty Russell <rusty@rustcorp.com.au> writes:

> Anthony Liguori <aliguori@us.ibm.com> writes:
>> We were memcpy()'ing a structure to the wire :-/  Since savevm really
>> only works on x86 today, lets just declare that this element is sent
>> over the wire as a little endian value in order to fix the bitness.
>>
>> Unfortunately, we also send raw pointers and size_t which are going
>> to be different values on a 32-bit vs. 64-bit QEMU so we need to also
>> deal with that case.
>>
>> A lot of values that should have been previously ignored are now sent
>> as 0 and ignored on the receive side too.
>
> Don't we want to transition to vmstate anyway?  Can we just do that, and
> relegate the existing slightly broken code, to legacy?

What worries me is if someone changes VirtQueueElement, then all the
sudden migration breaks.  By transitioning to what I've sent, we at
least have a better documented protocol that isn't prone to subtle
breakage anymore.  Plus, we resolve the endian issue before it becomes a
bigger problem when David actually gets live migration working reliably
on PPC...

I'm certainly in favor of cleaning up the savevm format and probably
leaving the existing load/save functions as-is for legacy purposes.
I'll leave that as an exercise for someone else though :-)

Regards,

Anthony Liguori

>
> Cheers,
> Rusty.