linux-kernel - [PATCH 12/16] x86/kvmclock: Mark TSC as reliable when it's constant and nonstop

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Message-ID: <20250201021718.699411-13-seanjc@google.com>
Date: Fri, 31 Jan 2025 18:17:14 -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>, 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>, 
	Alexey Makhalov <alexey.amakhalov@...adcom.com>, Jan Kiszka <jan.kiszka@...mens.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Andy Lutomirski <luto@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev, 
	virtualization@...ts.linux.dev, linux-hyperv@...r.kernel.org, 
	jailhouse-dev@...glegroups.com, kvm@...r.kernel.org, 
	xen-devel@...ts.xenproject.org, Sean Christopherson <seanjc@...gle.com>, 
	Nikunj A Dadhania <nikunj@....com>, Tom Lendacky <thomas.lendacky@....com>
Subject: [PATCH 12/16] x86/kvmclock: Mark TSC as reliable when it's constant
 and nonstop

Mark the TSC as reliable if the hypervisor (KVM) has enumerated the TSC
as constant and nonstop, and the admin hasn't explicitly marked the TSC
as unstable.  Like most (all?) virtualization setups, any secondary
clocksource that's used as a watchdog is guaranteed to be less reliable
than a constant, nonstop TSC, as all clocksources the kernel uses as a
watchdog are all but guaranteed to be emulated when running as a KVM
guest.  I.e. any observed discrepancies between the TSC and watchdog will
be due to jitter in the watchdog.

This is especially true for KVM, as the watchdog clocksource is usually
emulated in host userspace, i.e. reading the clock incurs a roundtrip
cost of thousands of cycles.

Marking the TSC reliable addresses a flaw where the TSC will occasionally
be marked unstable if the host is under moderate/heavy load.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kernel/kvmclock.c | 31 +++++++++++++++++--------------
 1 file changed, 17 insertions(+), 14 deletions(-)

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 890535ddc059..a7c4ae7f92e2 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -283,6 +283,7 @@ static int kvmclock_setup_percpu(unsigned int cpu)
 
 void __init kvmclock_init(void)
 {
+	enum tsc_properties tsc_properties = TSC_FREQUENCY_KNOWN;
 	u8 flags;
 
 	if (!kvm_para_available() || !kvmclock)
@@ -313,11 +314,26 @@ void __init kvmclock_init(void)
 	if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT))
 		pvclock_set_flags(PVCLOCK_TSC_STABLE_BIT);
 
+	/*
+	 * X86_FEATURE_NONSTOP_TSC is TSC runs at constant rate
+	 * with P/T states and does not stop in deep C-states.
+	 *
+	 * Invariant TSC exposed by host means kvmclock is not necessary:
+	 * can use TSC as clocksource.
+	 *
+	 */
+	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
+	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
+	    !check_tsc_unstable()) {
+		kvm_clock.rating = 299;
+		tsc_properties = TSC_FREQ_KNOWN_AND_RELIABLE;
+	}
+
 	flags = pvclock_read_flags(&hv_clock_boot[0].pvti);
 	kvm_sched_clock_init(flags & PVCLOCK_TSC_STABLE_BIT);
 
 	tsc_register_calibration_routines(kvm_get_tsc_khz, kvm_get_tsc_khz,
-					  TSC_FREQUENCY_KNOWN);
+					  tsc_properties);
 
 	x86_platform.get_wallclock = kvm_get_wallclock;
 	x86_platform.set_wallclock = kvm_set_wallclock;
@@ -328,19 +344,6 @@ void __init kvmclock_init(void)
 	x86_platform.restore_sched_clock_state = kvm_restore_sched_clock_state;
 	kvm_get_preset_lpj();
 
-	/*
-	 * X86_FEATURE_NONSTOP_TSC is TSC runs at constant rate
-	 * with P/T states and does not stop in deep C-states.
-	 *
-	 * Invariant TSC exposed by host means kvmclock is not necessary:
-	 * can use TSC as clocksource.
-	 *
-	 */
-	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
-	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
-	    !check_tsc_unstable())
-		kvm_clock.rating = 299;
-
 	clocksource_register_hz(&kvm_clock, NSEC_PER_SEC);
 	pv_info.name = "KVM";
 }
-- 
2.48.1.362.g079036d154-goog