linux-kernel - [PATCH v2 35/38] x86/kvmclock: Get TSC frequency from CPUID when its available

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Message-ID: <20250227021855.3257188-36-seanjc@google.com>
Date: Wed, 26 Feb 2025 18:18:51 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Sean Christopherson <seanjc@...gle.com>, Juergen Gross <jgross@...e.com>, 
	"K. Y. Srinivasan" <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, 
	Dexuan Cui <decui@...rosoft.com>, Ajay Kaher <ajay.kaher@...adcom.com>, 
	Jan Kiszka <jan.kiszka@...mens.com>, Andy Lutomirski <luto@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Daniel Lezcano <daniel.lezcano@...aro.org>, 
	John Stultz <jstultz@...gle.com>
Cc: linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev, 
	kvm@...r.kernel.org, virtualization@...ts.linux.dev, 
	linux-hyperv@...r.kernel.org, xen-devel@...ts.xenproject.org, 
	Tom Lendacky <thomas.lendacky@....com>, Nikunj A Dadhania <nikunj@....com>
Subject: [PATCH v2 35/38] x86/kvmclock: Get TSC frequency from CPUID when its available

When kvmclock and CPUID.0x15 are both present, use the TSC frequency from
CPUID.0x15 instead of kvmclock's frequency.  Barring a misconfigured
setup, both sources should provide the same frequency, CPUID.0x15 is
arguably a better source when using the TSC over kvmclock, and most
importantly, using CPUID.0x15 will allow stuffing the local APIC timer
frequency based on the core crystal frequency, i.e. will allow skipping
APIC timer calibration.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kernel/kvmclock.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index c45b321533e5..3efb837c7406 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -188,6 +188,16 @@ void kvmclock_cpu_action(enum kvm_guest_cpu_action action)
 	}
 }
 
+static unsigned long kvm_get_tsc_khz(void)
+{
+	struct cpuid_tsc_info info;
+
+	if (!cpuid_get_tsc_freq(&info))
+		return info.tsc_khz;
+
+	return pvclock_tsc_khz(this_cpu_pvti());
+}
+
 static unsigned long kvm_get_cpu_khz(void)
 {
 	unsigned int cpu_khz;
@@ -211,11 +221,6 @@ static unsigned long kvm_get_cpu_khz(void)
  * poll of guests can be running and trouble each other. So we preset
  * lpj here
  */
-static unsigned long kvm_get_tsc_khz(void)
-{
-	return pvclock_tsc_khz(this_cpu_pvti());
-}
-
 static void __init kvm_get_preset_lpj(void)
 {
 	unsigned long khz;
-- 
2.48.1.711.g2feabab25a-goog