Re: [PATCH 1/1] x86/hyperv: Set X86_FEATURE_TSC_KNOWN_FREQ when Hyper-V provides frequency

[Roman Kisel <romank@linux.microsoft.com>]

On 6/5/2024 7:55 PM, mhkelley58@gmail.com wrote:
> From: Michael Kelley <mhklinux@outlook.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@outlook.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@linux.microsoft.com>

-- 
Thank you,
Roman