linux-kernel - Re: [PATCH 3/3] perf, x86: Add INST_RETIRED.PREC

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Date:	Wed, 13 Aug 2014 10:10:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, eranian@...gle.com,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 3/3] perf, x86: Add INST_RETIRED.PREC_DIST workarounds

On Tue, Aug 12, 2014 at 06:45:15PM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> On Broadwell INST_RETIRED.PREC_DIST cannot be used with any period
> that doesn't have the lowest 6 bits cleared. And the period
> should not be smaller than 128.
> 
> Add a new callback to enforce this, and set it for Broadwell.
> 
> This is erratum BDM57 and BDM11.

Urgh, this blows.

> +/*
> + * Broadwell:
> + * The INST_RETIRED.ALL period always needs to have lowest
> + * 6bits cleared (BDM57). It shall not use a period smaller
> + * than 100 (BDM11). We combine the two to enforce
> + * a min-period of 128.
> + */
> +static unsigned bdw_limit_period(struct perf_event *event, unsigned left)
> +{
> +	if ((event->hw.config & 0xffff) == 0x1c0) {

ISTR we have actual proper defines for that.

> +		if (left < 128)
> +			left = 128;
> +		left &= ~0x3fu;
> +	}
> +	return left;
> +}

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