linux-kernel - Re: [Xen-devel] [PATCH] xen: update machine_to_phys

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Message-Id: <4E201CE8020000780004D8F0@nat28.tlf.novell.com>
Date:	Fri, 15 Jul 2011 09:56:40 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Ian Campbell" <Ian.Campbell@...rix.com>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>
Cc:	"Olaf Hering" <olaf@...fle.de>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Keir Fraser" <keir@....org>
Subject: Re: [Xen-devel] [PATCH] xen: update machine_to_phys_order on
	 resume

>>> On 13.07.11 at 15:20, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
> On Wed, Jul 13, 2011 at 10:12:44AM +0100, Ian Campbell wrote:
>> On Tue, 2011-07-12 at 19:11 +0100, Konrad Rzeszutek Wilk wrote:
>> > On Tue, Jul 12, 2011 at 06:43:42PM +0200, Olaf Hering wrote:
>> > > 
>> > > Migration of pv guests fails, the guest crashes on the target host once 
> the
>> > > guest is unpaused after transit. It happens when the guest is started on a
>> > > small systen, then migrated from that small system to a large system.
>> > > If the guest is started on a large system, then migrated to a small system 
> and
>> > > back to the large system, the migration will be successful.
>> > > 
>> > > The issue is that mfn_to_pfn() makes use of machine_to_phys_order, which
>> > > is only configured once early in the boot process. After migration to a
>> > > large host the mfns will exceed the order from the small system and a
>> > > wrong code path is taken.
>> > > 
>> > > Calling xen_setup_machphys_mapping() again in the resume path will avoid
>> > > the crash.
>> > 
>> > Oh, duh!
>> > 
>> > Let me queue that up for 3.0-rc7 unless there are objections?
>> 
>> It's not so much an objection to this patch but this issue seems to have
>> been caused by Xen cset 20892:d311d1efc25e which looks to me like a
>> subtle ABI breakage for guests. Perhaps we should introduce a feature
>> flag to indicate that a guest can cope with the m2p changing size over
>> migration like this?
> 
> Sounds reasonable to me.. I will wait (I can always submit it during 3.1 
> cycle
> and CC stable@...nel.org to backport it to 3.0.1).
> 
> Jan, you are the one who came up with the c/s - what's your thought?
> How does your kernel handle the changing size of the M2P - like the patch 
> below?

As said in an earlier reply to Ian, I didn't pay attention to the
migration aspects and I'm in favor of introducing a feature flag
to control the behavior.

In the meantime, as an immediate fix, I just sent out a patch to
revert to original behavior for all but Dom0.

Jan

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