From: Thomas Gleixner <tglx@kernel.org>
To: Sean Christopherson <seanjc@google.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, Kiryl Shutsemau <kas@kernel.org>,
Sean Christopherson <seanjc@google.com>,
"K. Y. Srinivasan" <kys@microsoft.com>,
Haiyang Zhang <haiyangz@microsoft.com>,
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Long Li <longli@microsoft.com>,
Ajay Kaher <ajay.kaher@broadcom.com>,
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Cc: "H. Peter Anvin" <hpa@zytor.com>,
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Nikunj A Dadhania <nikunj@amd.com>,
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Michael Kelley <mhklinux@outlook.com>
Subject: Re: [PATCH v4 01/47] x86/tsc: Never re-calibrate TSC frequency if its exact timing is known
Date: Fri, 05 Jun 2026 14:33:34 +0200 [thread overview]
Message-ID: <87fr315fq9.ffs@fw13> (raw)
In-Reply-To: <20260529144435.704127-2-seanjc@google.com>
On Fri, May 29 2026 at 07:43, Sean Christopherson wrote:
> Don't re-calibrate the TSC frequency if the TSC is known to run at a fixed
> frequency.
That's misleading because fixed frequency means that the frequency does
not change, i.e. X86_FEATURE_CONSTANT_TSC is set. But
X86_FEATURE_CONSTANT_TSC does not imply that the frequency can be read
from CPUID/MSRs.
> In practice, this is likely one big nop, as re-calibration is
> used only for SMP=n kernels, and only for hardware that is 20+ years old,
> i.e. is extremely unlikely to collide with TSC_KNOWN_FREQ.
recalibrate_cpu_khz() is only invoked from Intel P4 and AMD K7 CPU
frequency drivers, which means that's absolutely not interesting and
neither X86_FEATURE_CONSTANT_TSC nor X86_FEATURE_TSC_KNOWN_FREQ can be
set on those systems.
IOW, this patch is pointless voodoo ware.
Thanks,
tglx
next prev parent reply other threads:[~2026-06-05 12:33 UTC|newest]
Thread overview: 83+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-29 14:43 [PATCH v4 00/47] x86: Try to wrangle PV clocks vs. TSC Sean Christopherson
2026-05-29 14:43 ` [PATCH v4 01/47] x86/tsc: Never re-calibrate TSC frequency if its exact timing is known Sean Christopherson
2026-05-30 3:07 ` Borislav Petkov
2026-06-01 21:46 ` [PATCH v4 1/47] " David Woodhouse
2026-06-05 12:33 ` Thomas Gleixner [this message]
2026-06-05 18:04 ` [PATCH v4 01/47] " Sean Christopherson
2026-06-05 19:51 ` Thomas Gleixner
2026-05-29 14:43 ` [PATCH v4 02/47] x86/tsc: Add a standalone helpers for getting TSC info from CPUID.0x15 Sean Christopherson
2026-06-02 3:49 ` Borislav Petkov
2026-06-05 12:37 ` Thomas Gleixner
2026-05-29 14:43 ` [PATCH v4 03/47] x86/sev: Mark TSC as reliable when configuring Secure TSC Sean Christopherson
2026-05-29 14:43 ` [PATCH v4 04/47] x86/sev: Don't override CPU frequency calibration for SNP's " Sean Christopherson
2026-05-29 15:44 ` sashiko-bot
2026-05-29 14:43 ` [PATCH v4 05/47] x86/sev: Move check for SNP Secure TSC support to tsc_early_init() Sean Christopherson
2026-05-29 14:43 ` [PATCH v4 06/47] x86/sev: Shove SNP's secure/trusted TSC frequency directly into "calibration" Sean Christopherson
2026-05-29 16:14 ` sashiko-bot
2026-05-29 16:23 ` Sean Christopherson
2026-05-29 14:43 ` [PATCH v4 07/47] x86/tdx: Force TSC frequency with CPUID-based info provided by the TDX-Module Sean Christopherson
2026-05-29 16:21 ` sashiko-bot
2026-05-29 16:59 ` Sean Christopherson
2026-06-03 10:02 ` Kiryl Shutsemau
2026-05-29 14:43 ` [PATCH v4 08/47] x86/tsc: Add dedicated hypervisor hooks for getting known TSC/CPU frequencies Sean Christopherson
2026-06-01 21:49 ` [PATCH v4 8/47] " David Woodhouse
2026-05-29 14:43 ` [PATCH v4 09/47] x86/acrn: Mark TSC frequency as known when using ACRN for calibration Sean Christopherson
2026-05-29 16:40 ` sashiko-bot
2026-05-29 17:01 ` Sean Christopherson
2026-05-29 14:43 ` [PATCH v4 10/47] x86/tsc: Consolidate forcing of X86_FEATURE_TSC_KNOWN_FREQ for PV code Sean Christopherson
2026-05-29 19:01 ` sashiko-bot
2026-05-29 14:43 ` [PATCH v4 11/47] x86/tsc: Kill off x86_platform_ops.calibrate_{cpu,tsc}() hooks Sean Christopherson
2026-06-01 21:51 ` David Woodhouse
2026-05-29 14:43 ` [PATCH v4 12/47] x86/tsc: Rename pit_hpet_ptimer_calibrate_cpu() => native_calibrate_cpu_late() Sean Christopherson
2026-06-01 21:52 ` David Woodhouse
2026-05-29 14:44 ` [PATCH v4 13/47] x86/tsc: Fold native_calibrate_cpu() into recalibrate_cpu_khz() Sean Christopherson
2026-06-01 21:52 ` David Woodhouse
2026-05-29 14:44 ` [PATCH v4 14/47] x86/kvmclock: Rename kvm_get_tsc_khz() to kvmclock_get_tsc_khz() Sean Christopherson
2026-06-01 21:53 ` David Woodhouse
2026-05-29 14:44 ` [PATCH v4 15/47] KVM: x86: Officially define CPUID 0x40000010 as PV Timing Info (TSC and Bus) Sean Christopherson
2026-05-29 14:44 ` [PATCH v4 16/47] x86/kvm: Obtain TSC frequency from PV CPUID if present Sean Christopherson
2026-05-29 14:44 ` [PATCH v4 17/47] x86/kvm: Mark TSC as reliable when it's constant and nonstop Sean Christopherson
2026-05-29 18:12 ` sashiko-bot
2026-05-29 18:57 ` Sean Christopherson
2026-06-01 22:02 ` David Woodhouse
2026-05-29 14:44 ` [PATCH v4 18/47] x86/kvm: Get local APIC bus frequency from PV CPUID Timing Info Sean Christopherson
2026-05-29 18:12 ` sashiko-bot
2026-05-29 18:24 ` Sean Christopherson
2026-06-01 22:06 ` David Woodhouse
2026-05-29 14:44 ` [PATCH v4 19/47] x86/tsc: Add standalone helper for getting CPU frequency from CPUID Sean Christopherson
2026-05-29 14:44 ` [PATCH v4 20/47] x86/kvm: Get CPU base frequency from CPUID when it's available Sean Christopherson
2026-05-30 6:24 ` sashiko-bot
2026-05-29 14:44 ` [PATCH v4 21/47] x86/xen: Obtain TSC frequency from CPUID if present Sean Christopherson
2026-05-30 6:35 ` sashiko-bot
2026-05-29 14:44 ` [PATCH v4 22/47] clocksource: hyper-v: Register sched_clock save/restore iff it's necessary Sean Christopherson
2026-05-29 14:44 ` [PATCH v4 23/47] clocksource: hyper-v: Drop wrappers to sched_clock save/restore helpers Sean Christopherson
2026-05-29 14:44 ` [PATCH v4 24/47] clocksource: hyper-v: Don't save/restore TSC offset when using HV sched_clock Sean Christopherson
2026-05-29 14:44 ` [PATCH v4 25/47] x86/kvmclock: Setup kvmclock for secondary CPUs iff CONFIG_SMP=y Sean Christopherson
2026-05-29 14:44 ` [PATCH v4 26/47] x86/kvm: Don't disable kvmclock on BSP in syscore_suspend() Sean Christopherson
2026-05-30 7:08 ` sashiko-bot
2026-05-29 15:06 ` [PATCH v4 27/47] x86/paravirt: Remove unnecessary PARAVIRT=n stub for paravirt_set_sched_clock() Sean Christopherson
2026-05-29 15:07 ` [PATCH v4 28/47] x86/paravirt: Move handling of unstable PV clocks into paravirt_set_sched_clock() Sean Christopherson
2026-05-29 15:07 ` [PATCH v4 29/47] x86/kvmclock: Move sched_clock save/restore helpers up in kvmclock.c Sean Christopherson
2026-05-29 15:07 ` [PATCH v4 30/47] x86/xen/time: NOP-ify x86_platform's sched_clock save/restore hooks Sean Christopherson
2026-06-01 22:09 ` David Woodhouse
2026-05-29 15:07 ` [PATCH v4 31/47] x86/vmware: NOP-ify save/restore hooks when using VMware's sched_clock Sean Christopherson
2026-06-01 22:09 ` David Woodhouse
2026-05-29 15:07 ` [PATCH v4 32/47] x86/tsc: WARN if TSC sched_clock save/restore used with PV sched_clock Sean Christopherson
2026-05-29 15:07 ` [PATCH v4 33/47] x86/paravirt: Pass sched_clock save/restore helpers during registration Sean Christopherson
2026-05-29 15:08 ` [PATCH v4 34/47] x86/kvmclock: Move kvm_sched_clock_init() down in kvmclock.c Sean Christopherson
2026-05-29 15:08 ` [PATCH v4 35/47] x86/xen/time: Mark xen_setup_vsyscall_time_info() as __init Sean Christopherson
2026-05-29 15:08 ` [PATCH v4 36/47] x86/pvclock: Mark setup helpers and related various as __init/__ro_after_init Sean Christopherson
2026-05-29 15:08 ` [PATCH v4 37/47] x86/pvclock: WARN if pvclock's valid_flags are overwritten Sean Christopherson
2026-05-29 15:08 ` [PATCH v4 38/47] x86/kvmclock: Refactor handling of PVCLOCK_TSC_STABLE_BIT during kvmclock_init() Sean Christopherson
2026-05-29 15:08 ` [PATCH v4 39/47] timekeeping: Resume clocksources before reading persistent clock Sean Christopherson
2026-05-29 15:08 ` [PATCH v4 40/47] x86/kvmclock: Hook clocksource.suspend/resume when kvmclock isn't sched_clock Sean Christopherson
2026-05-29 15:08 ` [PATCH v4 41/47] x86/kvmclock: WARN if wall clock is read while kvmclock is suspended Sean Christopherson
2026-05-29 15:08 ` [PATCH v4 42/47] x86/paravirt: Mark __paravirt_set_sched_clock() as __init Sean Christopherson
2026-05-29 15:08 ` [PATCH v4 43/47] x86/paravirt: Plumb a return code into __paravirt_set_sched_clock() Sean Christopherson
2026-05-29 15:08 ` [PATCH v4 44/47] x86/paravirt: Don't use a PV sched_clock in CoCo guests with trusted TSC Sean Christopherson
2026-05-29 15:08 ` [PATCH v4 45/47] x86/kvmclock: Use TSC for sched_clock if it's constant and non-stop Sean Christopherson
2026-05-29 15:08 ` [PATCH v4 46/47] x86/kvmclock: Plumb in AP-online and BSP-resume to kvmlock, for documentation Sean Christopherson
2026-06-01 22:09 ` David Woodhouse
2026-05-29 15:08 ` [PATCH v4 47/47] x86/paravirt: Move using_native_sched_clock() stub into timer.h Sean Christopherson
2026-05-29 15:10 ` [PATCH v4 00/47] x86: Try to wrangle PV clocks vs. TSC Sean Christopherson
2026-05-29 15:17 ` Jürgen Groß
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