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Date:   Fri, 22 Mar 2019 16:00:09 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc:     x86 <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Len Brown <len.brown@intel.com>,
        Linux PM <linux-pm@vger.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Laura Abbott <labbott@fedoraproject.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Simon Schricker <sschricker@suse.de>,
        Borislav Petkov <bp@suse.de>, Hannes Reinecke <hare@suse.de>
Subject: Re: [PATCH 2/2] PM / arch: x86: MSR_IA32_ENERGY_PERF_BIAS sysfs
 interface
Message-ID: <20190322150009.GY6058@hirez.programming.kicks-ass.net>
References: <1637073.gl2OfxWTjI@aspire.rjw.lan>
 <1762575.ER2xjzr9E1@aspire.rjw.lan>
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On Thu, Mar 21, 2019 at 11:20:17PM +0100, Rafael J. Wysocki wrote:
> +	ret = rdmsrl_on_cpu(cpu, MSR_IA32_ENERGY_PERF_BIAS, &epb);
> +	if (ret < 0)
> +		return ret;
> +
> +	ret = wrmsrl_on_cpu(cpu, MSR_IA32_ENERGY_PERF_BIAS,
> +			    (epb & ~EPB_MASK) | val);

That's two back-to-back IPIs and a giant waste.

If you'd use a proper msr shadow variable, you'd not have to do the
rdmsr_on_cpu :-)