[tip:x86/urgent] x86/cpu: Use indirect call to measure performance in init_amd_k6()

[tip-bot for Mikulas Patocka]

Commit-ID:  5f8a16156aa1b2d0223eaee9dacdfb9bc096f610
Gitweb:     http://git.kernel.org/tip/5f8a16156aa1b2d0223eaee9dacdfb9bc096f610
Author:     Mikulas Patocka <mpatocka@xxxxxxxxxx>
AuthorDate: Tue, 11 Jul 2017 07:44:05 -0400
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Sun, 16 Jul 2017 11:05:04 +0200

x86/cpu: Use indirect call to measure performance in init_amd_k6()

This old piece of code is supposed to measure the performance of indirect
calls to determine if the processor is buggy or not, however the compiler
optimizer turns it into a direct call.

Use the OPTIMIZER_HIDE_VAR() macro to thwart the optimization, so that a real
indirect call is generated.

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/alpine.LRH.2.02.1707110737530.8746@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/kernel/cpu/amd.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index bb5abe8..3b9e220 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -134,6 +134,7 @@ static void init_amd_k6(struct cpuinfo_x86 *c)
 
 		n = K6_BUG_LOOP;
 		f_vide = vide;
+		OPTIMIZER_HIDE_VAR(f_vide);
 		d = rdtsc();
 		while (n--)
 			f_vide();