linux-kernel - [PATCH] x86/head/64: Skip fixmap page fixup

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Message-Id: <20161125111448.23623-1-bp@alien8.de>
Date:   Fri, 25 Nov 2016 12:14:48 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     X86 ML <x86@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] x86/head/64: Skip fixmap page fixup

From: Borislav Petkov <bp@...e.de>

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 <bp@...e.de>
---
 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