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x86/entry/64/compat: Document sysenter_fix_flags's reason for existence 

The code under the label can normally be inline, without the
jumping back and forth but the latter is an optimization.

Document that.

Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
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: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20151009170859.GA24266@pd.tnic
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Borislav Petkov 2015-10-09 19:08:59 +02:00 committed by Ingo Molnar
parent f5e6a9753a
commit 374a3a3916
1 changed files with 6 additions and 0 deletions
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arch/x86/entry/entry_64_compat.S
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@ -102,6 +102,12 @@ ENTRY(entry_SYSENTER_compat)
* NT was set instead of doing an unconditional popfq.
* This needs to happen before enabling interrupts so that
* we don't get preempted with NT set.
*
* NB.: sysenter_fix_flags is a label with the code under it moved
* out-of-line as an optimization: NT is unlikely to be set in the
* majority of the cases and instead of polluting the I$ unnecessarily,
* we're keeping that code behind a branch which will predict as
* not-taken and therefore its instructions won't be fetched.
*/
testl $X86_EFLAGS_NT, EFLAGS(%rsp)
jnz sysenter_fix_flags