Re: [Xen-devel] [PATCH] xen/mm: do direct hypercall in xen_set_pte() if batching is unavailable

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Jun 01, 2012 at 04:14:54PM +0100, David Vrabel wrote:
> From: David Vrabel <david.vrabel@xxxxxxxxxx>
> 
> In xen_set_pte() if batching is unavailable (because the caller is in
> an interrupt context such as handling a page fault) it would fall back
> to using native_set_pte() and trapping and emulating the PTE write.
> 
> On 32-bit guests this requires two traps for each PTE write (one for
> each dword of the PTE).  Instead, do one mmu_update hypercall
> directly.

OK.
> 
> This significantly improves page fault performance in 32-bit PV
> guests.

Nice!
> 
> lmbench3 test  Before    After     Improvement
> ----------------------------------------------
> lat_pagefault  3.18 us   2.32 us   27%
> lat_proc fork  356 us    313.3 us  11%
> 
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
>  arch/x86/xen/mmu.c |   16 ++++++++++++++--
>  1 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index b8e2794..3bf5dfa 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -308,8 +308,20 @@ static bool xen_batched_set_pte(pte_t *ptep, pte_t 
> pteval)
>  
>  static inline void __xen_set_pte(pte_t *ptep, pte_t pteval)
>  {
> -     if (!xen_batched_set_pte(ptep, pteval))
> -             native_set_pte(ptep, pteval);
> +     if (!xen_batched_set_pte(ptep, pteval)) {
> +             /*
> +              * Could call native_set_pte() here and trap and
> +              * emulate the PTE write but with 32-bit guests this
> +              * needs two traps (one for each of the two 32-bit
> +              * words in the PTE) so do one hypercall directly
> +              * instead.

Ouch.
> +              */
> +             struct mmu_update u;
> +
> +             u.ptr = virt_to_machine(ptep).maddr | MMU_NORMAL_PT_UPDATE;
> +             u.val = pte_val_ma(pteval);
> +             HYPERVISOR_mmu_update(&u, 1, NULL, DOMID_SELF);
> +     }
>  }
>  
>  static void xen_set_pte(pte_t *ptep, pte_t pteval)
> -- 
> 1.7.2.5
> 
> 
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