linux-kernel - Re: [PATCH] x86: Add a turbo mode sysctl

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Date:	Sun, 3 Apr 2016 11:55:34 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andy Lutomirski <luto@...capital.net>
cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] x86: Add a turbo mode sysctl

On Sat, 2 Apr 2016, Andy Lutomirski wrote:
> On Fri, Apr 1, 2016 at 9:10 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >> +     if (turbo_mode)
> >> +             write_cr0(cr0 & ~X86_CR0_CD);
> >> +     else
> >> +             write_cr0(cr0 | X86_CR0_CD);
> >
> > I think proper turbo mode disable requires ~(X86_CR0_CD | X86_CR0_NW)
> 
> I thought that made no difference on family 6 and P4 and was actively
> dangerous (disabled coherency) before.  At least, that's what the
> table of caching modes and the footnote seems to say.

CD=0 and NW=1 result in #GP, which might be your intent as that is definitely
the slowest mode you can achieve. But you should add a comment at least.

> I think we should merge this patch and add a special-case so that
> calling unlink on turbo_mode sets it to zero.  Then people who rm -rf
> / will brick their systems more slowly :)

While at it can you please make a turbo = 0 call when we hit a WARN/BUG/Panic
so the messages spill slower over the screen?

Thanks,

	tglx