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Date: Wed, 6 Sep 2017 16:14:37 +0100
From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Message-ID: <20170906151436.GF2215@work-vm>
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Subject: Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Peter Xu <peterx@redhat.com>, qemu-devel@nongnu.org, Laurent Vivier <lvivier@redhat.com>, Fam Zheng <famz@redhat.com>, Juan Quintela <quintela@redhat.com>, Markus Armbruster <armbru@redhat.com>, mdroth@linux.vnet.ibm.com, Paolo Bonzini <pbonzini@redhat.com>

* Stefan Hajnoczi (stefanha@gmail.com) wrote:
> On Wed, Aug 23, 2017 at 02:51:03PM +0800, Peter Xu wrote:
> > The root problem is that, monitor commands are all handled in main
> > loop thread now, no matter how many monitors we specify. And, if main
> > loop thread hangs due to some reason, all monitors will be stuck.
> 
> I see a larger issue with postcopy: existing QEMU code assumes that
> guest memory access is instantaneous.
> 
> Postcopy breaks this assumption and introduces blocking points that can
> now take unbounded time.
> 
> This problem isn't specific to the monitor.  It can also happen to other
> components in QEMU like the gdbstub.
> 
> Do we need an asynchronous memory API?  Synchronous memory access should
> only be allowed in vcpu threads.

It would probably be useful for gdbstub where the overhead of async
doesn't matter;  but doing that for all IO emulation is hard.

Dave

> Stefan
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK