linux-kernel - [PATCH v2 33/38] x86/kvmclock: Mark TSC as reliable when it's constant and nonstop

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250227021855.3257188-34-seanjc@google.com>
Date: Wed, 26 Feb 2025 18:18:49 -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 33/38] 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 | 29 ++++++++++++++++-------------
 1 file changed, 16 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index ce676e735ced..b924b19e8f0f 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -362,6 +362,7 @@ static void __init kvm_sched_clock_init(bool stable)
 
 void __init kvmclock_init(void)
 {
+	enum tsc_properties tsc_properties = TSC_FREQUENCY_KNOWN;
 	bool stable = false;
 
 	if (!kvm_para_available() || !kvmclock)
@@ -400,18 +401,6 @@ void __init kvmclock_init(void)
 			 PVCLOCK_TSC_STABLE_BIT;
 	}
 
-	kvm_sched_clock_init(stable);
-
-	tsc_register_calibration_routines(kvm_get_tsc_khz, kvm_get_tsc_khz,
-					  TSC_FREQUENCY_KNOWN);
-
-	x86_platform.get_wallclock = kvm_get_wallclock;
-	x86_platform.set_wallclock = kvm_set_wallclock;
-#ifdef CONFIG_SMP
-	x86_cpuinit.early_percpu_clock_init = kvm_setup_secondary_clock;
-#endif
-	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.
@@ -422,8 +411,22 @@ void __init kvmclock_init(void)
 	 */
 	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
 	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
-	    !check_tsc_unstable())
+	    !check_tsc_unstable()) {
 		kvm_clock.rating = 299;
+		tsc_properties = TSC_FREQ_KNOWN_AND_RELIABLE;
+	}
+
+	kvm_sched_clock_init(stable);
+
+	tsc_register_calibration_routines(kvm_get_tsc_khz, kvm_get_tsc_khz,
+					  tsc_properties);
+
+	x86_platform.get_wallclock = kvm_get_wallclock;
+	x86_platform.set_wallclock = kvm_set_wallclock;
+#ifdef CONFIG_SMP
+	x86_cpuinit.early_percpu_clock_init = kvm_setup_secondary_clock;
+#endif
+	kvm_get_preset_lpj();
 
 	clocksource_register_hz(&kvm_clock, NSEC_PER_SEC);
 	pv_info.name = "KVM";
-- 
2.48.1.711.g2feabab25a-goog