Re: [Xen-devel] [PATCH] [shadow] Disable higher level pagetables early unshadow only when the "process dying" hypercall is used.

[George Dunlap <dunlapg@xxxxxxxxx>]

On Wed, Nov 16, 2011 at 1:44 PM, Gianluca Guida <glguida@xxxxxxxxx> wrote:
> This patch fixes a performance problem in fully virtualized guests.

What performance problem, and how does it fix it?  The c/s doesn't
have any information about either, so it's hard for me to evaluate
whether I should pull it into the XenServer patchqueue...

>
> Should be backported to xen-4.0 and xen-4.1. This patch applies (with
> slightly different offsets) to all three versions.
>
> Signed-off-by: Gianluca Guida <gianluca.guida@xxxxxxxxxx>
> Tested-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> diff -r 068d3d55ce6e xen/arch/x86/mm/shadow/multi.c
> --- a/xen/arch/x86/mm/shadow/multi.c    Tue Nov 01 19:03:38 2011 +0000
> +++ b/xen/arch/x86/mm/shadow/multi.c    Wed Nov 16 14:30:57 2011 -0800
> @@ -2723,8 +2723,9 @@
>            || ( !v->domain->arch.paging.shadow.pagetable_dying_op
>                 &&
> v->arch.paging.shadow.last_emulated_mfn_for_unshadow == mfn_x(gmfn) )
> )
>          && sh_mfn_is_a_page_table(gmfn)
> -         && !(mfn_to_page(gmfn)->shadow_flags
> -              & (SHF_L2_32|SHF_L2_PAE|SHF_L2H_PAE|SHF_L4_64)) )
> +         && (!v->domain->arch.paging.shadow.pagetable_dying_op ||
> +             !(mfn_to_page(gmfn)->shadow_flags
> +               & (SHF_L2_32|SHF_L2_PAE|SHF_L2H_PAE|SHF_L4_64))) )
>     {
>         perfc_incr(shadow_early_unshadow);
>         sh_remove_shadows(v, gmfn, 1, 0 /* Fast, can fail to unshadow */ );
>
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