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 <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

0 files changed, 0 insertions, 0 deletions