Re: [Xen-devel] [PATCH] xen: update machine_to_phys_order on resume

[Ian Campbell <Ian.Campbell@citrix.com>]

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?

Ian.

> 
> > 
> > Signed-off-by: Olaf Hering <olaf@aepfle.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();
> > 
> > _______________________________________________
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> > Xen-devel@lists.xensource.com
> > http://lists.xensource.com/xen-devel
> 
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