linux-kernel - Re: [PATCH] x86, tsc add an initial read offset to __cycles_2

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Date:	Fri, 26 Jul 2013 12:26:22 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
	Dave Hansen <dave@...1.net>, x86@...nel.org
Subject: Re: [PATCH] x86, tsc add an initial read offset to __cycles_2_ns()
 calculations

On Wed, Jul 24, 2013 at 12:03:20PM -0400, Prarit Bhargava wrote:
> +/*
> + * TSC can have non-zero values at boot time on Intel Xeon E5 (family 6,
> + * model 45) aka "SandyBridge" processors.  This is documented in the
> + * Errata for the processors as BT81.  As a result, we need to snapshot
> + * the TSC's initial value to avoid calculation overflows in the conversions
> + * of cycles to nanoseconds.
> + */
> +unsigned long long tsc_initial_value;
> +
>  /*
>   * Scheduler clock - returns current time in nanosec units.
>   */
> @@ -979,6 +989,9 @@ void __init tsc_init(void)
>  		return;
>  	}
>  
> +	tsc_initial_value = get_cycles();
> +	pr_info("TSC: tsc initial value = %lld\n", tsc_initial_value);
> +

And we probably don't need that info on processors unaffected by the
erratum...

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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