[tip:x86/asm] x86/entry/64/compat: Document sysenter_fix_flags' s reason for existence

[tip-bot for Borislav Petkov]

Commit-ID:  374a3a3916a70fc6236bc2b8f8ac02548a128a54
Gitweb: http://git.kernel.org/tip/374a3a3916a70fc6236bc2b8f8ac02548a128a54
Author: Borislav Petkov 
AuthorDate: Fri, 9 Oct 2015 19:08:59 +0200
Committer:  Ingo Molnar 
CommitDate: Sun, 11 Oct 2015 11:06:40 +0200

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.ga24...@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 cf9641c..92b0b27 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
--
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[tip:x86/asm] x86/entry/64/compat: Document sysenter_fix_flags' s reason for existence

[tip-bot for Borislav Petkov]

Commit-ID:  374a3a3916a70fc6236bc2b8f8ac02548a128a54
Gitweb: http://git.kernel.org/tip/374a3a3916a70fc6236bc2b8f8ac02548a128a54
Author: Borislav Petkov 
AuthorDate: Fri, 9 Oct 2015 19:08:59 +0200
Committer:  Ingo Molnar 
CommitDate: Sun, 11 Oct 2015 11:06:40 +0200

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.ga24...@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 cf9641c..92b0b27 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
--
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