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Re: [Qemu-devel] [PATCH 2/6] Introduce monitor 'wait' command (v2)

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH 2/6] Introduce monitor 'wait' command (v2)
Date: Thu, 09 Apr 2009 08:40:22 -0500
User-agent: Thunderbird (X11/20090320)

Avi Kivity wrote:
Anthony Liguori wrote:
The wait command will pause the monitor the command was issued in until a new event becomes available. Events are queued if there isn't a waiter present.
The wait command completes after a single event is available.

How do you stop a wait if there are no pending events?

You mean, cancel a wait? You cannot. I thought about whether it was a problem or not. I'm not sure.

You could introduce a wait-cancel command, but then you need a way to identify which wait you want to cancel. I can't think of a simple way to do that today.

Today, we queue events indefinitely but in the future, I suspect we'll drop
events that are older than a certain amount of time to avoid infinitely
allocating memory for long running VMs.

This queueing plug the race where an event happens immediately after a wait completes. But it could be avoided completely by having asynchronous notifications on a single monitor.

There are multiple things I think are being confused: asynchronous completion of monitor commands, events, monitor multiplexing, and non-human mode.

There can only be one command active at any given time on a Monitor context. We can have many Monitor contexts. There is currently only one Monitor context connected to a character device at a given time.

I think what you want to see is something like this:

<input> tag=4: info cpus
<input> tag=5: info kvm
<output> tag=5,rc=0: kvm enabled
<output> tag=4,rc=0: eip = 0x0000000444
<ouput> rc=0,class=vm-state,name=start: vm started

To me, this is a combination of events, which is introduced by this patch, a non-human monitor mode, and finally multiplexing multiple monitors into a single session.

Right now, you can have the same functional thing by having three monitors. In the first monitor you'd see:

(qemu) info cpus
eip = 0x000000444

In the second you'd see:

(qemu) info kvm
kvm enabled

In the third you'd see:

(qemu) wait
23423423.23423: vm-state: start: vm started

Even those the two info commands today are synchronous, there's nothing requiring them to be (see migrate as an example). So I think we're in agreement but you just want to jump ahead 6 months ;-)


Anthony Liguori

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