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

From: Avi Kivity
Subject: Re: [Qemu-devel] [PATCH 1/6] Allow multiple monitor devices (v2)
Date: Thu, 09 Apr 2009 11:19:43 +0300
User-agent: Thunderbird (X11/20090320)

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.

What's wrong with having 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.

Multiple monitors would still come in useful for debugging in a managed environment.

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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