From: Borislav Petkov <b...@suse.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 <b...@suse.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
