From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 45FD2346FB3; Thu, 21 May 2026 21:38:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779399487; cv=none; b=ffLIkge/2LLyV88w6xilCCMoAuX/i9JZyZVksRyeh10gVDZ4iIIpfdhz7vT7aE39KTUKxhVRkfnhohOsy94l9zHlasgh+wDgLxnZU9E1IehVkitG2YWnWEuf3tZJ4Edkk6pdOM1QWxvALp35ohZaiaLeQAh51WrZSZ/Xa4Os6WI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779399487; c=relaxed/simple; bh=naYJSgMs80K4M4tUqy478SUb9QHxon8J9AdYNto51PY=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=KWCfl/cxBIXauNSw6RoqWoyxEGMu1JnKG7BQ0vr/XeukXShG90VwV32QbgaKPKC+O3YKhIKmf3luOJoQCHY9wGyif+0WiwES6PQzYiOaDlAoLscpr8dlxIm3Y6ZdPt9/vIvNE1SBqRRyC4qm6GUKCJoXQDEPknD64MVKBMDJnkI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=casper.srs.infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=LXuT3nYE; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=casper.srs.infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="LXuT3nYE" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=MIME-Version:Content-Type:References: In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=sto4vrdGp1xVl4xxluQsgqhSN4UEkQ/MLv3WN0D9yfk=; b=LXuT3nYEMlTaLuHFZsDAwQQvD9 pfHkh+x/71+8eRSzLdbvIb+OkRxVn+tiRFMYGWHM7wM8LBg1x2bFQ+M5AHHQrhQeQMW0MPHL5wvYy RUqo2JBmVRWytz/ctqc2HPucGt82uc+lZyOMeI71JDmbBiynF84n7QBT3V1RoVmA1Ij91D73WiRra It3PtIKXpGwPeoVfphgLTvCkr8EnW66WQua5aZbvBpPUumSBqmcmrLkCiwtg+PR4/TAiaPLgYMft7 VsfAGiDbelE3Slzj1KKF5w1MGZC5JtqkO4SxcNNPBF+kUIbeMPkkB7+6Oi5X77Z82swELNGdJuL8D z9VxHj8A==; Received: from 54-240-197-239.amazon.com ([54.240.197.239] helo=u09cd745991455d.ant.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wQB5W-000000094VX-0s0t; Thu, 21 May 2026 21:37:56 +0000 Message-ID: Subject: Re: [PATCH v3 02/41] x86/tsc: Add helper to register CPU and TSC freq calibration routines From: David Woodhouse To: Sean Christopherson Cc: Kiryl Shutsemau , Paolo Bonzini , "K. Y. Srinivasan" , Haiyang Zhang , Wei Liu , Dexuan Cui , Long Li , Ajay Kaher , Alexey Makhalov , Jan Kiszka , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Juergen Gross , Daniel Lezcano , Thomas Gleixner , John Stultz , Rick Edgecombe , Vitaly Kuznetsov , Broadcom internal kernel review list , Boris Ostrovsky , Stephen Boyd , x86@kernel.org, linux-coco@lists.linux.dev, kvm@vger.kernel.org, linux-hyperv@vger.kernel.org, virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org, Michael Kelley , Tom Lendacky , Nikunj A Dadhania , Thomas Gleixner Date: Thu, 21 May 2026 22:37:52 +0100 In-Reply-To: References: <20260515191942.1892718-1-seanjc@google.com> <20260515191942.1892718-3-seanjc@google.com> <44e0d60548d317fd59895f18bd17220dfb2f834b.camel@infradead.org> <342098f6bfe1e4c7b233433df8f79713b4220614.camel@infradead.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-0JYzVT5JMNJwWPKngpYY" User-Agent: Evolution 3.52.3-0ubuntu1.1 Precedence: bulk X-Mailing-List: linux-hyperv@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org. See http://www.infradead.org/rpr.html --=-0JYzVT5JMNJwWPKngpYY Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2026-05-21 at 14:17 -0700, Sean Christopherson wrote: > =C2=A0 > > That seems reasonable. Where does the call to > > native_calibrate_tsc() > > happen; is that from determine_cpu_tsc_frequencies()?=20 >=20 > Yep. Great, thanks. > static bool __init determine_cpu_tsc_frequencies(bool early, > unsigned int > known_cpu_khz, > unsigned int > known_tsc_khz) > { > /* Make sure that cpu and tsc are not already calibrated */ > WARN_ON(cpu_khz || tsc_khz); >=20 > if (early) { > /* > * Early CPU calibration can only use methods that > are available > * early in boot (obviously). > */ > if (known_cpu_khz) > cpu_khz =3D known_cpu_khz; > else > cpu_khz =3D native_calibrate_cpu_early(); > if (known_tsc_khz) > tsc_khz =3D known_tsc_khz; > else > tsc_khz =3D native_calibrate_tsc(); > } else { > cpu_khz =3D pit_hpet_ptimer_calibrate_cpu(); > } If, after all that, we still end up in the case where we *do* have to calibrate it against a legacy timer (which sadly IIRC is the case even on some fairly modern AMD generations), could we round the answer? We currently have *far* more precision than accuracy, leading to values like 2399997kHz which change every boot (and end up being what gets *advertised* to guests on such a host... and then unless we're careful to avoid it, we end up trying to *scale* a different host's TSC down from 2399998 to 2399997 for a guest which is migrated from the first host...) We should just fix them (e.g. to 2400000kHz) and let NTP sort them out. Something like "round to the nearest MHz if that's within =C2=B110PPM"? --=-0JYzVT5JMNJwWPKngpYY Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCD9Aw ggSOMIIDdqADAgECAhAOmiw0ECVD4cWj5DqVrT9PMA0GCSqGSIb3DQEBCwUAMGUxCzAJBgNVBAYT AlVTMRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5jb20xJDAi BgNVBAMTG0RpZ2lDZXJ0IEFzc3VyZWQgSUQgUm9vdCBDQTAeFw0yNDAxMzAwMDAwMDBaFw0zMTEx MDkyMzU5NTlaMEExCzAJBgNVBAYTAkFVMRAwDgYDVQQKEwdWZXJva2V5MSAwHgYDVQQDExdWZXJv a2V5IFNlY3VyZSBFbWFpbCBHMjCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAMjvgLKj jfhCFqxYyRiW8g3cNFAvltDbK5AzcOaR7yVzVGadr4YcCVxjKrEJOgi7WEOH8rUgCNB5cTD8N/Et GfZI+LGqSv0YtNa54T9D1AWJy08ZKkWvfGGIXN9UFAPMJ6OLLH/UUEgFa+7KlrEvMUupDFGnnR06 aDJAwtycb8yXtILj+TvfhLFhafxroXrflspavejQkEiHjNjtHnwbZ+o43g0/yxjwnarGI3kgcak7 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