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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>, Wei Liu <wei.liu@kernel.org>, Dexuan Cui
 <decui@microsoft.com>, Long Li <longli@microsoft.com>, Ajay Kaher
 <ajay.kaher@broadcom.com>, Alexey Makhalov <alexey.makhalov@broadcom.com>,
 Jan Kiszka <jan.kiszka@siemens.com>, Andy Lutomirski <luto@kernel.org>,
 Peter Zijlstra <peterz@infradead.org>, Juergen Gross <jgross@suse.com>,
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 <jstultz@google.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Rick Edgecombe
 <rick.p.edgecombe@intel.com>, Vitaly Kuznetsov <vkuznets@redhat.com>,
 Broadcom internal kernel review list
 <bcm-kernel-feedback-list@broadcom.com>, Boris Ostrovsky
 <boris.ostrovsky@oracle.com>, Stephen Boyd <sboyd@kernel.org>,
 kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
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 Woodhouse <dwmw@amazon.co.uk>, Tom Lendacky <thomas.lendacky@amd.com>,
 Nikunj A Dadhania <nikunj@amd.com>, David Woodhouse <dwmw2@infradead.org>,
 Michael Kelley <mhklinux@outlook.com>
Subject: Re: [PATCH v4 01/47] x86/tsc: Never re-calibrate TSC frequency if
 its exact timing is known
In-Reply-To: <20260529144435.704127-2-seanjc@google.com>
References: <20260529144435.704127-1-seanjc@google.com>
 <20260529144435.704127-2-seanjc@google.com>
Date: Fri, 05 Jun 2026 14:33:34 +0200
Message-ID: <87fr315fq9.ffs@fw13>
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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