[PATCH] x86/kernel/cpu/amd.c: use indirect call

[PATCH] x86/kernel/cpu/amd.c: use indirect call

[Mikulas Patocka]

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 macro OPTIMIZER_HIDE_VAR to thwart the optimization, so that real 
indirect call is generated.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 arch/x86/kernel/cpu/amd.c |    1 +
 1 file changed, 1 insertion(+)

Index: linux-2.6/arch/x86/kernel/cpu/amd.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/cpu/amd.c
+++ linux-2.6/arch/x86/kernel/cpu/amd.c
@@ -134,6 +134,7 @@ static void init_amd_k6(struct cpuinfo_x
 
 		n = K6_BUG_LOOP;
 		f_vide = vide;
+		OPTIMIZER_HIDE_VAR(f_vide);
 		d = rdtsc();
 		while (n--)
 			f_vide();

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

[tip-bot for Mikulas Patocka]

Commit-ID:  429ed86b5c99b388543f6f7f6983b1230326f2bb
Gitweb:     http://git.kernel.org/tip/429ed86b5c99b388543f6f7f6983b1230326f2bb
Author:     Mikulas Patocka <mpatocka@redhat.com>
AuthorDate: Tue, 11 Jul 2017 07:44:05 -0400
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 12 Jul 2017 10:42:32 +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@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
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: Josh Poimboeuf <jpoimboe@redhat.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/alpine.LRH.2.02.1707110737530.8746@file01.intranet.prod.int.rdu2.redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 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();

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

[Borislav Petkov]

On Wed, Jul 12, 2017 at 06:09:57AM -0700, tip-bot for Mikulas Patocka wrote:
> Commit-ID:  429ed86b5c99b388543f6f7f6983b1230326f2bb
> Gitweb:     http://git.kernel.org/tip/429ed86b5c99b388543f6f7f6983b1230326f2bb
> Author:     Mikulas Patocka <mpatocka@redhat.com>
> AuthorDate: Tue, 11 Jul 2017 07:44:05 -0400
> Committer:  Ingo Molnar <mingo@kernel.org>
> CommitDate: Wed, 12 Jul 2017 10:42:32 +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.

Are you booting on a real K6 or qemu?

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
--

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

[Mikulas Patocka]

On Wed, 12 Jul 2017, Borislav Petkov wrote:

> On Wed, Jul 12, 2017 at 06:09:57AM -0700, tip-bot for Mikulas Patocka wrote:
> > Commit-ID:  429ed86b5c99b388543f6f7f6983b1230326f2bb
> > Gitweb:     http://git.kernel.org/tip/429ed86b5c99b388543f6f7f6983b1230326f2bb
> > Author:     Mikulas Patocka <mpatocka@redhat.com>
> > AuthorDate: Tue, 11 Jul 2017 07:44:05 -0400
> > Committer:  Ingo Molnar <mingo@kernel.org>
> > CommitDate: Wed, 12 Jul 2017 10:42:32 +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.
> 
> Are you booting on a real K6 or qemu?

I tested it on K6-3 and it doesn't trigger the warning, the loop with the 
indirect call takes 9 million cycles (it took 4 million cycles with the 
direct call).

I don't have the old K6 revision with the bug.

Mikulas

> -- 
> Regards/Gruss,
>     Boris.
> 
> ECO tip #101: Trim your mails when you reply.
> --

[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@redhat.com>
AuthorDate: Tue, 11 Jul 2017 07:44:05 -0400
Committer:  Ingo Molnar <mingo@kernel.org>
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@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
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: Josh Poimboeuf <jpoimboe@redhat.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/alpine.LRH.2.02.1707110737530.8746@file01.intranet.prod.int.rdu2.redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 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();