linux-kernel - Re: [PATCH 1/2] x86/tsc: add X86_FEATURE_TSC_KNOWN

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Date:   Wed, 9 Nov 2016 22:09:38 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Bin Gao <bin.gao@...el.com>
cc:     Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        x86@...nel.org, Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/tsc: add X86_FEATURE_TSC_KNOWN_FREQ flag

On Tue, 1 Nov 2016, Bin Gao wrote:

> The X86_FEATURE_TSC_RELIABLE flag in Linux kernel implies both reliable
> (at runtime) and trustable (at calibration). But reliable running and
> trustable calibration are logically irrelevant. Per Thomas Gleixner's
> suggestion we would like to split this flag into two separate flags:
> X86_FEATURE_TSC_RELIABLE - running reliably
> X86_FEATURE_TSC_KNOWN_FREQ - frequency is known (no calibration required)
> These two flags allow Linux kernel to act differently based on
> processor/SoC's capability, i.e. no watchdog on TSC if TSC is reliable,
> and no calibration if TSC frequency is known.
> 
> Current Linux kernel already gurantees calibration is skipped for
> processors that can report TSC frequency by CPUID or MSR. However, the
> delayed calibration is still not skipped for these CPUID/MSR capable
> processors. The new flag X86_FEATURE_TSC_KNOWN_FREQ added by this patch
> will gurantee the delayed calibration is skipped.
> 
> Signed-off-by: Bin Gao <bin.gao@...el.com>
> ---
>  arch/x86/include/asm/cpufeatures.h | 1 +
>  arch/x86/kernel/tsc.c              | 6 +++---
>  2 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index a396292..7f6a5f8 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -106,6 +106,7 @@
>  #define X86_FEATURE_APERFMPERF	( 3*32+28) /* APERFMPERF */
>  #define X86_FEATURE_EAGER_FPU	( 3*32+29) /* "eagerfpu" Non lazy FPU restore */
>  #define X86_FEATURE_NONSTOP_TSC_S3 ( 3*32+30) /* TSC doesn't stop in S3 state */
> +#define X86_FEATURE_TSC_KNOWN_FREQ ( 3*32+31) /* TSC has known frequency */
>  
>  /* Intel-defined CPU features, CPUID level 0x00000001 (ecx), word 4 */
>  #define X86_FEATURE_XMM3	( 4*32+ 0) /* "pni" SSE-3 */
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 46b2f41..b4c82f8 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -1283,10 +1283,10 @@ static int __init init_tsc_clocksource(void)
>  		clocksource_tsc.flags |= CLOCK_SOURCE_SUSPEND_NONSTOP;
>  
>  	/*
> -	 * Trust the results of the earlier calibration on systems
> -	 * exporting a reliable TSC.
> +	 * When TSC frequency is known (generally got by MSR or CPUID), we skip
> +	 * the refined calibration and directly register it as a clocksource.
>  	 */
> -	if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE)) {
> +	if (boot_cpu_has(X86_FEATURE_TSC_KNOWN_FREQ)) {

This causes a regression, because with only this patch applied the
architectures which use the reliable flag for this today are not longer
taking this path.

The proper thing to do here is to make this:

	if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE) ||
	    boot_cpu_has(X86_FEATURE_TSC_KNOWN_FREQ)) {

and remove the RELIABLE flag ckeck after the existing users are converted.

Thanks,

	tglx