linux-kernel - Re: [PATCH 1/1] x86/hyperv: Set X86_FEATURE_TSC_KNOWN

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Message-ID: <226804eb-af9d-4a56-aef5-e3045e83b551@linux.microsoft.com>
Date: Tue, 11 Jun 2024 07:51:48 -0700
From: Roman Kisel <romank@...ux.microsoft.com>
To: mhklinux@...look.com, kys@...rosoft.com, haiyangz@...rosoft.com,
 wei.liu@...nel.org, decui@...rosoft.com, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
 hpa@...or.com, linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] x86/hyperv: Set X86_FEATURE_TSC_KNOWN_FREQ when
 Hyper-V provides frequency



On 6/5/2024 7:55 PM, mhkelley58@...il.com wrote:
> From: Michael Kelley <mhklinux@...look.com>
> 
> A Linux guest on Hyper-V gets the TSC frequency from a synthetic MSR, if
> available. In this case, set X86_FEATURE_TSC_KNOWN_FREQ so that Linux
> doesn't unnecessarily do refined TSC calibration when setting up the TSC
> clocksource.
> 
> With this change, a message such as this is no longer output during boot
> when the TSC is used as the clocksource:
> 
> [    1.115141] tsc: Refined TSC clocksource calibration: 2918.408 MHz
> 
> Furthermore, the guest and host will have exactly the same view of the
> TSC frequency, which is important for features such as the TSC deadline
> timer that are emulated by the Hyper-V host.
> 
> Signed-off-by: Michael Kelley <mhklinux@...look.com>
> ---
>   arch/x86/kernel/cpu/mshyperv.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index e0fd57a8ba84..c3e38eaf6d2f 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -424,6 +424,7 @@ static void __init ms_hyperv_init_platform(void)
>   	    ms_hyperv.misc_features & HV_FEATURE_FREQUENCY_MSRS_AVAILABLE) {
>   		x86_platform.calibrate_tsc = hv_get_tsc_khz;
>   		x86_platform.calibrate_cpu = hv_get_tsc_khz;
> +		setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
>   	}
>   
>   	if (ms_hyperv.priv_high & HV_ISOLATION) {

LGTM

Reviewed-by: Roman Kisel <romank@...ux.microsoft.com>

-- 
Thank you,
Roman