x86/entry/64/compat: Document sysenter_fix_flags's reason for existence
authorBorislav Petkov <bp@alien8.de>
Fri, 9 Oct 2015 17:08:59 +0000 (19:08 +0200)
committerIngo Molnar <mingo@kernel.org>
Sun, 11 Oct 2015 09:06:40 +0000 (11:06 +0200)
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>
arch/x86/entry/entry_64_compat.S

index cf9641cd47965bd3c7ee7b04b142914da25cb69c..92b0b27b43c694e2dadc550ac2d65588a509dc17 100644 (file)
@@ -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