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Re: [Qemu-devel] [PATCH 1/6] Allow multiple monitor devices (v2)

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH 1/6] Allow multiple monitor devices (v2)
Date: Thu, 09 Apr 2009 08:28:39 -0500
User-agent: Thunderbird (X11/20090320)

Avi Kivity wrote:
Anthony Liguori wrote:
Right now only one monitor device can be enabled at a time. In order to support asynchronous notification of events, I would like to introduce a 'wait' command that waits for an event to occur. This implies that we need an additional monitor session to allow commands to still be executed while waiting for an
asynchronous notification.

I think this is race prone.  For example:

 monitor 1: wait

 monitor 2: hotunplug dev1
 monitor 2: hotplug dev1

 monitor 1: event

there is no way to tell whether event (which relates to dev1) happened the hotunplug or after the hotunplug. In general there is no way to correlate events to commands.

events don't correlate to commands. Can you give an example of why you'd want to correlate and event to a command.

What's wrong with having async notifications?

Isn't this async notifications?

Sure, we'll need to make sure notifications don't mix with command responses, and that all commands are acked (the (qemu) prompt serves that purpose now), but it seems to me do be a lot easier for the management end.

FWIW, you can have asynchronous completion of monitor commands. See migration as an example. The (qemu) prompt is the ack that the command is finished. You can only have one async command per monitor session which is why you need multiple monitors.

If we had a non-human monitor mode, I think it would be fine to have many monitors multiplexed over a single channel. The internal monitor abstraction doesn't change.

I think multiplexing multiple monitor sessions requires a non-human mode because you need to tag all output, etc. which is what we need for non-human mode anyway.


Anthony Liguori

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