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Subject[tip:x86/boot] x86/boot/64: Optimize fixmap page fixup
Commit-ID:  6248f4567442081994ad61c63bd9870e147983e0
Gitweb: http://git.kernel.org/tip/6248f4567442081994ad61c63bd9870e147983e0
Author: Borislav Petkov <bp@suse.de>
AuthorDate: Fri, 25 Nov 2016 12:14:48 +0100
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 28 Nov 2016 07:45:17 +0100

x86/boot/64: Optimize fixmap page fixup

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@suse.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20161125111448.23623-1-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
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 b4421cc..a4a814f 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -147,6 +147,9 @@ 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
@@ -154,9 +157,9 @@ 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 */
@@ -167,6 +170,7 @@ 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)
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