On Thu, 26 Jan 2012 08:18:03 -0700
Eric Blake<address@hidden> 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.
Note that this is only about specific features that depend on host support,
like S3 suspend which is known to be buggy in current and old qemu.
But whose job is it to tell the guest agent what version of qemu is
running? Based on the above conversation, it looks like the current
qemu implementation does not do any handshaking on its own when the
guest agent first comes alive, which means that you are forcing the work
on the management app (libvirt). And this is inherently racy - if the
guest is allowed to restart its qemu-ga process at will, and each
restart of that guest process triggers a need to redo the handshake,
then libvirt can never reliably know what version the agent is running at.
Making the set-support-level persistent would solve it, wouldn't it?