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

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

[Gianluca Guida]

This patch fixes a performance problem in fully virtualized guests.

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@citrix.com>
Tested-by: Jan Beulich <jbeulich@suse.com>

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 */ );

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

[George Dunlap]

On Wed, Nov 16, 2011 at 1:44 PM, Gianluca Guida <glguida@gmail.com> 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@citrix.com>
> Tested-by: Jan Beulich <jbeulich@suse.com>
>
> 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 */ );
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel

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

[Gianluca Guida]

Hi George,

On Wed, Dec 14, 2011 at 7:58 AM, George Dunlap <dunlapg@umich.edu> wrote:
> On Wed, Nov 16, 2011 at 1:44 PM, Gianluca Guida <glguida@gmail.com> 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...

Have a look at this:

http://old-list-archives.xen.org/archives/html/xen-devel/2011-11/msg00650.html

A good question is whether the performance improvement that brought
Stefano to implement the hypercall in Linux was due to the original
patch creating a regression or if it really helps even after the
performance regression is fixed.

I wasn't here on the Xen world while the patch was tested on unstable,
so i have no idea about it.

Gianluca

-- 
It was a type of people I did not know, I found them very strange and
they did not inspire confidence at all. Later I learned that I had been
introduced to electronic engineers.
                                                  E. W. Dijkstra