linux-kernel - Re: x86/tsc: Set X86_FEATURE_TSC_RELIABLE to skip refined calibration

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Fri, 17 Jun 2016 09:48:13 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Bin Gao <bin.gao@...ux.intel.com>
cc:	Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
	bin.gao@...el.com, John Stultz <john.stultz@...aro.org>
Subject: Re: x86/tsc: Set X86_FEATURE_TSC_RELIABLE to skip refined
 calibration

On Thu, 16 Jun 2016, Bin Gao wrote:

> Unlike PIT based calibration which counts TSC cycles against another timer,
> MSR or CPUID method has no calibration - it simply multiplies the known
> frequency of a timer by a ratio. So TSC frequency computed by MSR or CPUID
> is the final frequency and doesn't need the refined calibration process.
> We used to use set_cpu_cap(&boot_cpu_data, X86_FEATURE_TSC_RELIABLE) but
> it actually doesn't skip refined calibration because the flag is cleared
> later in identify_cpu(). A cpu caps flag is not cleared only if it's set
> by setup_force_cpu_cap(). This patch sets the flag in tsc_msr.c and
> replaces set_cpu_cap() with setup_force_cpu_cap() in other files.

I'm not entirely sure that this is correct. At least I want to know John
Stultz's opinion on that.
 
Thanks,

	tglx