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Re: [Qemu-devel] [PATCH 0/2] add writeconfig command on monitor
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH 0/2] add writeconfig command on monitor |
Date: |
Thu, 16 Feb 2017 09:53:22 +0000 |
User-agent: |
Mutt/1.7.1 (2016-10-04) |
* Markus Armbruster (address@hidden) wrote:
> Eduardo Otubo <address@hidden> writes:
>
> > This first patch extends the command line option `-writeconfig <file>' to a
> > command on HMP and QMP monitors. This is useful when live migrating after a
> > series of device hot plug events. One can just generate an updated config
> > file
> > for the vm, transport it to the target host and start the vm with
> > `-readconfig
> > <file>'.
> >
> > The second patch re-includes the reference of the memory object on the
> > config
> > file generated.
>
> The high-level idea of having QEMU regurgitate its configuration for the
> migration target sounds nice, but there are several issues with
> regurgitating QemuOpts state with writeconfig:
>
> 1. Our needs have outgrown QemuOpts' design. We have accumulated
> various hacks and work-arounds to make do, and it's still not enough.
> Instead of adding more, I want to revise its design. The work has
> started, but it'll take some time. Adding creative new uses of
> QemuOpts while this work is in progress can only make it harder.
>
> If this issue was the only one, I'd take the hit for the team.
>
> 2. Transmitting configuration at the beginning of migration doesn't
> fully solve the problem. What about configuration changes during
> migration? Think of hot plug. Doesn't mean transmitting
> configuration is a bad idea, only means there's more to the problem
> than a naive observer might think.
>
> In my opinion, the proper solution is to transmit configuration
> information in the migration stream, complete with updates as it
> changes. Hard to do, which is why it hasn't been done.
>
> If we can't have the proper solution now, a less-than-ideal partial
> solution may still be better than nothing.
That's a separate problem from the one Eduardo is trying to solve;
I wouldn't trust migration to survive a device hotplugged during the migration
as it is. So I wouldn't worry about it as a reason against this series.
> 3. The accuracy of QemuOpts information is doubtful.
>
> Completeness: only certain kinds of configuration are done with
> QemuOpts. Incompleteness makes -writeconfig less useful than it
> could be, but it's still useful. Monitor command writeconfig could
> be similarly useful.
>
> Correctness: configuration gets stored in QemuOpts when we parse
> KEY=VALUE,... strings. It can also be constructed and updated
> manually. At certain points in time, bits from QemuOpts are used to
> actually configure stuff.
>
> Example: -device creates an entry in the "device" configuration
> group, which is later used to actually create and configure a device
> object.
>
> My point is: whenever we manipulate the actual objects, we may
> invalidate information stored in QemuOpts. We can try to keep it in
> sync, and we do at least sometimes. But this is a game we can only
> lose, except for the period(s) of time where QemuOpts is all there
> is, i.e. before actual objects get created. Note that -writeconfig
> runs before objects get created, so it's not affected by this issue.
>
> Out-of-sync QemuOpts is harmless unless something relies on it being
> accurate. I know we currently rely on QemuOpts IDs to catch
> duplicate IDs for some of the configuration groups. I doubt there's
> much else.
>
> If we add your monitor command, out-of-sync QemuOpts goes from
> harmless to serious bug. In other words, we'd create a new class of
> bugs, with an unknown number of existing instances that are probably
> hard to find and fix. Probably a perpetual source of new instances,
> too.
>
> Feels like a show stopper to me.
Hmm this does seem a bigger problem.
Dave
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK