[tip:x86/pti] x86/retpoline: Use LFENCE instead of PAUSE in the retpoline/RSB filling RSB macros

[tip-bot for Tom Lendacky <tipbot@zytor.com>]

Commit-ID:  2eb9137c8744f9adf1670e9aa52850948a30112b
Gitweb:     https://git.kernel.org/tip/2eb9137c8744f9adf1670e9aa52850948a30112b
Author:     Tom Lendacky <thomas.lendacky@amd.com>
AuthorDate: Fri, 12 Jan 2018 19:07:28 -0600
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Sat, 13 Jan 2018 11:28:50 +0100

x86/retpoline: Use LFENCE instead of PAUSE in the retpoline/RSB filling RSB macros

The PAUSE instruction is currently used in the retpoline and RSB filling
macros as a speculation trap.  The use of PAUSE was originally suggested
because it showed a very, very small difference in the amount of
cycles/time used to execute the retpoline as compared to LFENCE.

On AMD, the PAUSE instruction is not a serializing instruction, so the
PAUSE/JMP loop will use excess power as it is speculated over waiting
for return to mispredict to the correct target.

The RSB filling macro is applicable to AMD, and, if software is unable to
verify that LFENCE is serializing on AMD (possible when running under a
hypervisor), the generic retpoline support will be used and, so, is also
applicable to AMD.  Change the use of PAUSE to LFENCE.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Greg Kroah-Hartman <gregkh@linux-foundation.org>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kees Cook <keescook@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul Turner <pjt@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Link: http://lkml.kernel.org/r/20180113010728.27928.8537.stgit@tlendack-t1.amdoffice.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/include/asm/nospec-branch.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 402a11c..2c4a09a 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -11,7 +11,7 @@
  * Fill the CPU return stack buffer.
  *
  * Each entry in the RSB, if used for a speculative 'ret', contains an
- * infinite 'pause; jmp' loop to capture speculative execution.
+ * infinite 'lfence; jmp' loop to capture speculative execution.
  *
  * This is required in various cases for retpoline and IBRS-based
  * mitigations for the Spectre variant 2 vulnerability. Sometimes to
@@ -37,12 +37,12 @@
 771:						\
 	call	772f;				\
 773:	/* speculation trap */			\
-	pause;					\
+	lfence;					\
 	jmp	773b;				\
 772:						\
 	call	774f;				\
 775:	/* speculation trap */			\
-	pause;					\
+	lfence;					\
 	jmp	775b;				\
 774:						\
 	dec	reg;				\
@@ -72,7 +72,7 @@
 .macro RETPOLINE_JMP reg:req
 	call	.Ldo_rop_\@
 .Lspec_trap_\@:
-	pause
+	lfence
 	jmp	.Lspec_trap_\@
 .Ldo_rop_\@:
 	mov	\reg, (%_ASM_SP)
@@ -164,7 +164,7 @@
 	"       jmp    904f;\n"					\
 	"       .align 16\n"					\
 	"901:	call   903f;\n"					\
-	"902:	pause;\n"					\
+	"902:	lfence;\n"					\
 	"       jmp    902b;\n"					\
 	"       .align 16\n"					\
 	"903:	addl   $4, %%esp;\n"				\