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

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Date:	Wed, 13 Jul 2011 09:20:38 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
Cc:	Keir Fraser <keir@....org>, Jan Beulich <jbeulich@...ell.com>,
	Olaf Hering <olaf@...fle.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>
Subject: Re: [Xen-devel] [PATCH] xen: update machine_to_phys_order on resume

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?
> 
> Ian.
> 
> > 
> > > 
> > > Signed-off-by: Olaf Hering <olaf@...fle.de>
> > > 
> > > ---
> > >  arch/x86/xen/mmu.c     |    2 +-
> > >  arch/x86/xen/suspend.c |    2 ++
> > >  2 files changed, 3 insertions(+), 1 deletion(-)
> > > 
> > > Index: linux-3.0-rc7/arch/x86/xen/mmu.c
> > > ===================================================================
> > > --- linux-3.0-rc7.orig/arch/x86/xen/mmu.c
> > > +++ linux-3.0-rc7/arch/x86/xen/mmu.c
> > > @@ -1623,7 +1623,7 @@ static void __init xen_map_identity_earl
> > >  	set_page_prot(pmd, PAGE_KERNEL_RO);
> > >  }
> > >  
> > > -void __init xen_setup_machphys_mapping(void)
> > > +void xen_setup_machphys_mapping(void)
> > >  {
> > >  	struct xen_machphys_mapping mapping;
> > >  	unsigned long machine_to_phys_nr_ents;
> > > Index: linux-3.0-rc7/arch/x86/xen/suspend.c
> > > ===================================================================
> > > --- linux-3.0-rc7.orig/arch/x86/xen/suspend.c
> > > +++ linux-3.0-rc7/arch/x86/xen/suspend.c
> > > @@ -43,6 +43,8 @@ void xen_arch_hvm_post_suspend(int suspe
> > >  
> > >  void xen_arch_post_suspend(int suspend_cancelled)
> > >  {
> > > +	xen_setup_machphys_mapping();
> > > +
> > >  	xen_build_mfn_list_list();
> > >  
> > >  	xen_setup_shared_info();
> > > 
> > > _______________________________________________
> > > Xen-devel mailing list
> > > Xen-devel@...ts.xensource.com
> > > http://lists.xensource.com/xen-devel
> > 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@...ts.xensource.com
> > http://lists.xensource.com/xen-devel
> 
> 
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