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Re: [Qemu-devel] [PATCH RFC 1/8] virtio: add subsections to the migratio

From: Andreas Färber
Subject: Re: [Qemu-devel] [PATCH RFC 1/8] virtio: add subsections to the migration stream
Date: Thu, 15 May 2014 11:58:25 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

Am 15.05.2014 11:52, schrieb Michael S. Tsirkin:
> On Thu, May 15, 2014 at 11:20:18AM +0200, Andreas Färber wrote:
>> Am 15.05.2014 09:04, schrieb Greg Kurz:
>>> On Thu, 15 May 2014 12:16:35 +0530
>>> Amit Shah <address@hidden> wrote:
>>>> On (Thu) 15 May 2014 [09:23:51], Michael S. Tsirkin wrote:
>>>>> On Thu, May 15, 2014 at 11:34:25AM +0530, Amit Shah wrote:
>>>>>> On (Wed) 14 May 2014 [17:41:38], Greg Kurz wrote:
>>>>>>> Since each virtio device is streamed in its own section, the idea is to
>>>>>>> stream subsections between the end of the device section and the start
>>>>>>> of the next sections. This allows an older QEMU to complain and exit
>>>>>>> when fed with subsections:
>>>>>>> Unknown savevm section type 5
>>>>>>> Error -22 while loading VM state
>>>>>> Please make this configurable -- either via configure or device
>>>>>> properties.  That avoids having to break existing configurations that
>>>>>> work without this patch.
>> Since backwards migration is not supported upstream, wouldn't it be
>> easiest to just add support for the subsection marker and skipping to
>> the end of section in that downstream?
> Backwards and forwards migration need to be supported,
> customers told us repeatedly. So some downstreams support this
> and not supporting it upstream just means downstreams need
> to do their own thing.
> As importantly, ping-pong migration is the only
> reliable way to stress migration.
> So if we want to test cross-version we need it to work
> both way.
> Finally, the real issue and difficulty with cross-version migration is
> making VM behave in a backwards compatible way.  Serializing in a
> compatible way is a trivial problem, or would be if the code wasn't a
> mess :) Once you do the hard part, breaking migration because of the
> trivial serialization issue is just silly.  And special-casing forward
> migration does not make code simpler, it really only leads to
> proliferation of hacks and lack of symmetry.
> So yes it's a useful feature, and no not supporting it does
> not help anyway.

It seems you misunderstand. I was not saying it's not useful.

My point is that VMStateSubsections added in newer versions (and thus
not existing in older versions) need to be handled for any
VMState-converted devices. So why can't we make that work for virtio too?


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