From mboxrd@z Thu Jan 1 00:00:00 1970 From: Konrad Rzeszutek Wilk Subject: Re: [PATCH] xen/mm: do direct hypercall in xen_set_pte() if batching is unavailable Date: Tue, 5 Jun 2012 12:07:46 -0400 Message-ID: <20120605160746.GB24031@phenom.dumpdata.com> References: <1338563694-21915-1-git-send-email-david.vrabel@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <1338563694-21915-1-git-send-email-david.vrabel@citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: David Vrabel Cc: xen-devel@lists.xensource.com List-Id: xen-devel@lists.xenproject.org On Fri, Jun 01, 2012 at 04:14:54PM +0100, David Vrabel wrote: > From: David Vrabel > > 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 > --- > 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 > > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xen.org > http://lists.xen.org/xen-devel