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From: mhkelley58@gmail.com
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Subject: [PATCH 1/1] x86/hyperv: Set X86_FEATURE_TSC_KNOWN_FREQ when Hyper-V provides frequency
Date: Wed,  5 Jun 2024 19:55:59 -0700
Message-Id: <20240606025559.1631-1-mhklinux@outlook.com>
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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) {
-- 
2.25.1