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Re: [libvirt] [Qemu-devel] [PATCH RFC 0/4] Allow hibernation on guests

On 01/26/2012 01:35 PM, Luiz Capitulino wrote:
On Thu, 26 Jan 2012 08:18:03 -0700
Eric Blake<eblake redhat com>  wrote:

[adding qemu-devel]

On 01/26/2012 07:46 AM, Daniel P. Berrange wrote:
One thing, that you'll probably notice is this
'set-support-level' command. Basically, it tells GA what qemu version
is it running on. Ideally, this should be done as soon as
GA starts up. However, that cannot be determined from outside
world as GA doesn't emit any events yet.
Ideally^2 this command should be left out as it should be qemu
who tells its own agent this kind of information.
Anyway, I was going to call this command in qemuProcess{Startup,
Reconnect,Attach}, but it won't work. We need to un-pause guest CPUs
so guest can boot and start GA, but that implies returning from qemuProcess*.

So I am setting this just before 'guest-suspend' command, as
there is one more thing about GA. It is unable to remember anything
upon its restart (GA process). Which has BTW show flaw
in our current code with FS freeze&  thaw. If we freeze guest
FS, and somebody restart GA, the simple FS Thaw will not succeed as
GA thinks FS are not frozen. But that's a different cup of tea.

Because of what written above, we need to call set-level
on every suspend.

IMHO all this says that the 'set-level' command is a conceptually
unfixably broken design&  should be killed in QEMU before it turns
into an even bigger mess.

Can you elaborate on this? Michal and I talked on irc about making the
compatibility level persistent, would that help?

Once we're in a situation where we need to call 'set-level' prior
to every single invocation, you might as well just allow the QEMU
version number to be passed in directly as an arg to the command
you are running directly thus avoiding this horrificness.

Qemu folks, would you care to chime in on this?

Exactly how is the set-level command supposed to work?  As I understand
it, the goal is that if the guest has qemu-ga 1.1 installed, but is
being run by qemu 1.0, then we want to ensure that any guest agent
command supported by qemu-ga 1.1 but requiring features of qemu not
present in qemu 1.0 will be properly rejected.

Not exactly, the default support of qemu-ga is qemu 1.0. This means that by
default qemu-ga will only support qemu 1.0 even when running on qemu 2.0. This
way the set-support-level command allows you to specify that qemu 2.0 features
are supported.

Version numbers are meaningless. What happens when a bunch of features get backported by RHEL such that qemu-ga 1.0 ends up being a frankenstein version of 2.0?

The feature negotiation mechanism we have in QMP is the existence of a command. If we're in a position where we're trying to disable part of a command, it simply means that we should have multiple commands such that we can just remove the disabled part entirely.


Anthony Liguori

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