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

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Date:	Sat, 2 Apr 2016 13:21:41 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Thomas Gleixner <tglx@...utronix.de>
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 Fri, Apr 1, 2016 at 9:10 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Fri, 1 Apr 2016, Andy Lutomirski wrote:
>> +static void update_local_turbo_mode(void *dummy)
>> +{
>> +     unsigned long cr0 = read_cr0();
>> +
>> +     /*
>> +      * KVM doesn't properly handle CD.
>> +      *
>> +      * XXX: this may interact poorly with CPU hotplug.
>
> Please cc these crazy folks who cleanup the hotplug mess so they can put it on
> their todo list.
>
>> +      */
>> +
>> +     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.

>
>> +static void update_turbo_mode(void)
>> +{
>> +     on_each_cpu(update_local_turbo_mode, NULL, 1);
>> +
>> +     if (!turbo_mode)
>> +             wbinvd();
>
> You really want to do wbinvd() on each cpu to make sure that each cpu gets out
> of that turbo thing.

Nah, this is an explicit optimization to de-turboize as quickly as possible:

"The instruction then issues a special-function bus cycle that directs
external caches to also write
back modified data and another bus cycle to indicate that the external
caches should be invalidated."

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 :)

--Andy