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Re: [PATCH v4 01/47] x86/tsc: Never re-calibrate TSC frequency if its exact timing is known
- To: Sean Christopherson <seanjc@xxxxxxxxxx>
- From: Thomas Gleixner <tglx@xxxxxxxxxx>
- Date: Tue, 09 Jun 2026 21:27:17 +0200
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- Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, x86@xxxxxxxxxx, Kiryl Shutsemau <kas@xxxxxxxxxx>, "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>, Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>, Wei Liu <wei.liu@xxxxxxxxxx>, Dexuan Cui <decui@xxxxxxxxxxxxx>, Long Li <longli@xxxxxxxxxxxxx>, Ajay Kaher <ajay.kaher@xxxxxxxxxxxx>, Alexey Makhalov <alexey.makhalov@xxxxxxxxxxxx>, Jan Kiszka <jan.kiszka@xxxxxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>, John Stultz <jstultz@xxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>, Broadcom internal kernel review list <bcm-kernel-feedback-list@xxxxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Stephen Boyd <sboyd@xxxxxxxxxx>, kvm@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, linux-coco@xxxxxxxxxxxxxxx, linux-hyperv@xxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx, David Woodhouse <dwmw@xxxxxxxxxxxx>, Tom Lendacky <thomas.lendacky@xxxxxxx>, Nikunj A Dadhania <nikunj@xxxxxxx>, David Woodhouse <dwmw2@xxxxxxxxxxxxx>, Michael Kelley <mhklinux@xxxxxxxxxxx>
- Delivery-date: Tue, 09 Jun 2026 19:27:25 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Tue, Jun 09 2026 at 10:17, Sean Christopherson wrote:
> On Fri, Jun 05, 2026, Thomas Gleixner wrote:
>> On Fri, Jun 05 2026 at 11:04, Sean Christopherson wrote:
>> But we also should have a check in the TSC init code somewhere which
>> validates that X86_FEATURE_CONSTANT_TSC is set when
>> X86_FEATURE_TSC_KNOWN_FREQ is set. X86_FEATURE_TSC_KNOWN_FREQ is useless
>> w/o X86_FEATURE_CONSTANT_TSC.
>
> Ugh, any objection to punting on this for now? KVM and Xen guests will
> trigger
> TSC_KNOWN_FREQ without CONSTANT_TSC, thanks to commits:
>
> e10f78050323 ("kvmclock: fix TSC calibration for nested guests")
> 898ec52d2ba0 ("x86/xen/time: Set the X86_FEATURE_TSC_KNOWN_FREQ flag in
> xen_tsc_khz()")
>
> Hyper-V guests might as well? Hyper-V's handling of TSC is weird, even for a
> hypervisor.
Hypervisors are ranked by weirdness? I ranked them by insanity so far.
> Even when the frequency is provided in CPUID by the hypervisor, QEMU at least
> requires a fairly explicit opt-in to advertise CONSTANT_TSC, presumably to try
> to prevent users from shooting themselves in the foot.
Bah. We really should have enforced the dependency when we introduced
KNOWN_FREQ. But that ship has sailed.
Though for correctness sake this should be fixed at some point in the
foreseeable future.
Thanks,
tglx
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