From 374a3a3916a70fc6236bc2b8f8ac02548a128a54 Mon Sep 17 00:00:00 2001 From: Borislav Petkov Date: Fri, 9 Oct 2015 19:08:59 +0200 Subject: [PATCH] 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 Acked-by: Andy Lutomirski Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/20151009170859.GA24266@pd.tnic Signed-off-by: Ingo Molnar --- arch/x86/entry/entry_64_compat.S | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S index cf9641cd4796..92b0b27b43c6 100644 --- a/arch/x86/entry/entry_64_compat.S +++ b/arch/x86/entry/entry_64_compat.S @@ -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 -- 2.34.1