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

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Message-ID: <alpine.DEB.2.11.1604011803390.3978@nanos>
Date:	Fri, 1 Apr 2016 18:10:03 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andy Lutomirski <luto@...nel.org>
cc:	x86@...nel.org, Borislav Petkov <bp@...en8.de>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH] x86: Add a turbo mode sysctl

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)

> +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.

Thanks,

	tglx