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Re: [Xen-devel] [PATCH] Allow PV superpages to work with live migration

To: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] Allow PV superpages to work with live migration
From: Dave McCracken <dcm@xxxxxxxx>
Date: Tue, 18 May 2010 11:21:12 -0500
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Xen Developers List <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <Keir.Fraser@xxxxxxxxxxxxx>
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On Tuesday, May 18, 2010, Tim Deegan wrote:
> At 15:20 +0100 on 18 May (1274196018), Dave McCracken wrote:
> > I'm confused.  As far as I know, opt_allow_hugeage only affects PV
> > guests.  All I did in the macro was add a test for it.  I didn't touch
> > any of the HVM logic.  Are you saying the HVM logic is wrong or that
> > opt_allow_hugepage affects HVM guests in some fashion?
> It would affect them after your change:
> +    return (opt_allow_hugepage ||
> +           (is_hvm_vcpu(v) &&
> +            (GUEST_PAGING_LEVELS != 2
> +             || !hvm_paging_enabled(v)
> +             || (v->arch.hvm_vcpu.guest_cr[4] & X86_CR4_PSE))));
> This makes the whole predicate true _even_for_hvm_guests_ whenever
> opt_allow_hugepage is true.  It should not be true for HVM guests that
> have disabled PSE, regardless of the setting of opt_allow_hugepage.

Ahh, right.  It didn't occur to me that people might set that parameter on 
machines with HVM guests.  I suppose it would be reasonable if they had a mix 
of PV and HVM on the same machine.

On Tuesday, May 18, 2010, Keir wrote:
> I already fixed this when I applied the patch this morning.



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