Re: [Qemu-devel] [RFC v5 00/26] QMP: out-of-band (OOB) execution support

From: Stefan Hajnoczi <stefanha@redhat.com>
To: Fam Zheng <famz@redhat.com>
Cc: Peter Xu <peterx@redhat.com>,
	armbru@redhat.com, qemu-devel@nongnu.org,
	Laurent Vivier <lvivier@redhat.com>,
	Juan Quintela <quintela@redhat.com>,
	mdroth@linux.vnet.ibm.com, marcandre.lureau@redhat.com,
	Paolo Bonzini <pbonzini@redhat.com>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>
Subject: Re: [Qemu-devel] [RFC v5 00/26] QMP: out-of-band (OOB) execution support
Date: Fri, 15 Dec 2017 13:30:54 +0000	[thread overview]
Message-ID: <20171215133054.GI26982@stefanha-x1.localdomain> (raw)
In-Reply-To: <20171215104101.GE15187@lemon>

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On Fri, Dec 15, 2017 at 06:41:01PM +0800, Fam Zheng wrote:
> On Tue, 12/05 13:51, Peter Xu wrote:
> > This version is mostly document update, and dropped the single patch
> > that is migration related (will be put into postcopy recovery
> > series).
> 
> Sorry if I'm asking an already answered question, if so please remind me with
> some pointers..
> 
> Can we completely hide the "OOB" concept from QMP to avoid the interface
> complication? In other words, the new "migrate-recover" command can be
> implemented in a way that it always try to run out of band as if it has
> "run-oob=true", without user specifying it or even negotiating it.
> 
> Because it seems "migrate-recover" is only useful with "run-oob=true".
> 
> Of course without introducing the whole OOB concept in QMP, what this command
> does will sound very hacky, but the bahavior can be documented just well on the
> single command. I don't see a big advantage in generalizing OOB at the QMP level
> so far. Do we have a list of other commands to be made OOB?

No, that's not possible because the batching behavior changes between
oob enabled vs disabled:

Clients that don't enable oob can send several commands at once.  They
expect them to be reliably executed in sequence.

When oob is enabled batched commands can be dropped because the queue is
full.  This is necessary because otherwise the client can fill the
monitors socket receive buffer with normal commands and then it's
impossible to get an oob command through to the monitor.

Stefan

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