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Message-ID: <861f0a3e-8d6f-473e-a67d-80e46343fedd@default>
Date: Mon, 2 Nov 2009 07:28:21 -0800 (PST)
From: Dan Magenheimer <dan.magenheimer@...cle.com>
To: Avi Kivity <avi@...hat.com>
Cc: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
kurt.hackel@...cle.com, Glauber Costa <glommer@...hat.com>,
the arch/x86 maintainers <x86@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Glauber de Oliveira Costa <gcosta@...hat.com>,
Xen-devel <xen-devel@...ts.xensource.com>,
Keir Fraser <keir.fraser@...citrix.com>, zach.brown@...cle.com,
Ingo Molnar <mingo@...hat.com>, chris.mason@...cle.com
Subject: RE: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall
implementation
> From: Avi Kivity [mailto:avi@...hat.com]
>
> On 10/29/2009 06:15 PM, Dan Magenheimer wrote:
> > On a related note, though some topic drift, many of
> > the problems that occur in virtualization due to migration
> > could be better addressed if Linux had an architected
> > interface to allow it to be signaled if a migration
> > occurred, and if Linux could signal applications of
> > the same. I don't have any cycles (pun intended) to
> > think about this right now, but if anyone else starts
> > looking at it, I'd love to be cc'ed.
>
> IMO that's not a good direction. The hypervisor should not depend on
> the guest for migration (the guest may be broken, or
> malicious, or being
> debugged, or slow). So the notification must be asynchronous, which
> means that it will only be delivered to applications after
> migration has
> completed.
I definitely agree that the hypervisor can't wait for a guest
to respond.
You've likely thought through this a lot more than I have,
but I was thinking that if the kernel received the notification
as some form of interrupt, it could determine immediately
if any running threads had registered for "SIG_MIGRATE"
and deliver the signal synchronously.
> Instead of a "migration has occured, run for the hills" signal we're
> better of finding out why applications want to know about
> this event and
> addressing specific needs.
Perhaps. It certainly isn't warranted for this one
special case of timestamp handling. But I'll bet 5-10 years
from now, after we've handled a few special cases, we'll
wish that we would have handled it more generically.
Dan
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