From: David Vrabel Date: Fri, 1 Jun 2012 15:14:54 +0000 (+0100) Subject: xen/mm: do direct hypercall in xen_set_pte() if batching is unavailable X-Git-Tag: next-20120724~28^2~12^2 X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=c901f72c2b0a842530bef4542bc3f0409663c750;p=karo-tx-linux.git xen/mm: do direct hypercall in xen_set_pte() if batching is unavailable 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. This significantly improves page fault performance in 32-bit PV guests. 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 Signed-off-by: Konrad Rzeszutek Wilk --- diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c index b756d8cf4df5..a91030f158bd 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. + */ + 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)