linux-kernel - Re: [tip:x86/urgent] x86/cpu: Use indirect call to measure performance in init_amd

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Date:   Wed, 12 Jul 2017 22:20:12 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
cc:     linux-tip-commits@...r.kernel.org, brgerst@...il.com,
        dvlasenk@...hat.com, peterz@...radead.org, hpa@...or.com,
        tglx@...utronix.de, linux-kernel@...r.kernel.org, luto@...nel.org,
        jpoimboe@...hat.com, torvalds@...ux-foundation.org,
        mingo@...nel.org
Subject: Re: [tip:x86/urgent] x86/cpu: Use indirect call to measure performance
 in init_amd_k6()



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@...hat.com>
> > AuthorDate: Tue, 11 Jul 2017 07:44:05 -0400
> > Committer:  Ingo Molnar <mingo@...nel.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.
> --