From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752925AbcKYLO6 (ORCPT ); Fri, 25 Nov 2016 06:14:58 -0500 Received: from mail.skyhub.de ([78.46.96.112]:59132 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752084AbcKYLOv (ORCPT ); Fri, 25 Nov 2016 06:14:51 -0500 From: Borislav Petkov To: X86 ML Cc: LKML Subject: [PATCH] x86/head/64: Skip fixmap page fixup Date: Fri, 25 Nov 2016 12:14:48 +0100 Message-Id: <20161125111448.23623-1-bp@alien8.de> X-Mailer: git-send-email 2.10.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Borislav Petkov Single-stepping through head_64.S made me look at the fixmap page PTEs fixup loop: So we're going through the whole level2_fixmap_pgt 4K page, looking at whether PAGE_PRESENT is set in those PTEs and add the delta between where we're compiled to run and where we actually end up running. However, if that delta is 0 (most cases) we go through all those 512 PTEs for no reason at all. Oh well, we add 0 but that's no reason to me. Skipping that useless fixup gives us a boot speedup of 0.004 seconds in my guest. Not a lot but considering how cheap it is, I'll take it. Here is the printk time difference: before: ... [ 0.000000] tsc: Marking TSC unstable due to TSCs unsynchronized [ 0.013590] Calibrating delay loop (skipped), value calculated using timer frequency.. 8027.17 BogoMIPS (lpj=16054348) [ 0.017094] pid_max: default: 32768 minimum: 301 ... after: ... [ 0.000000] tsc: Marking TSC unstable due to TSCs unsynchronized [ 0.009587] Calibrating delay loop (skipped), value calculated using timer frequency.. 8026.86 BogoMIPS (lpj=16053724) [ 0.013090] pid_max: default: 32768 minimum: 301 ... For the other two changes converting naked numbers to defines: # arch/x86/kernel/head_64.o: text data bss dec hex filename 1124 290864 4096 296084 48494 head_64.o.before 1124 290864 4096 296084 48494 head_64.o.after md5: 87086e202588939296f66e892414ffe2 head_64.o.before.asm 87086e202588939296f66e892414ffe2 head_64.o.after.asm Signed-off-by: Borislav Petkov --- arch/x86/kernel/head_64.S | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S index a15d381e6020..90de28841242 100644 --- a/arch/x86/kernel/head_64.S +++ b/arch/x86/kernel/head_64.S @@ -142,6 +142,9 @@ startup_64: decl %ecx jnz 1b + test %rbp, %rbp + jz .Lskip_fixup + /* * Fixup the kernel text+data virtual addresses. Note that * we might write invalid pmds, when the kernel is relocated @@ -149,9 +152,9 @@ startup_64: * beyond _end. */ leaq level2_kernel_pgt(%rip), %rdi - leaq 4096(%rdi), %r8 + leaq PAGE_SIZE(%rdi), %r8 /* See if it is a valid page table entry */ -1: testb $1, 0(%rdi) +1: testb $_PAGE_PRESENT, 0(%rdi) jz 2f addq %rbp, 0(%rdi) /* Go to the next page */ @@ -162,6 +165,7 @@ startup_64: /* Fixup phys_base */ addq %rbp, phys_base(%rip) +.Lskip_fixup: movq $(early_level4_pgt - __START_KERNEL_map), %rax jmp 1f ENTRY(secondary_startup_64) -- 2.10.0