From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (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 E1EC44A35; Sat, 24 Jan 2026 01:31:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769218312; cv=none; b=P3jUxwZm0DclTWa2IVKagCv8DkjV3thAy3w9irrUK4+xMdPeDp3Uzn4LZtcQyE874dvghqGyQmqGCsgJV09zDAUqMeBk9rj7tzfKC5djmRnXpLHd+qnv47qy3eC5EJuSzKGvgamx/wGSRO027IxuTVJiDYythnWo0pi3nZ8km2I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769218312; c=relaxed/simple; bh=Z1Cr3VHZ2N5IQ8CUi2xKvTg3Y8erVu0IcZEsJQQswaY=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=ao54Hzid+kxt808k7oksQYVLFH+9PIO3neo5XpSCXTTamDH2bdbB6EITa2RuNFS1HQygW50pZadRV72PvVK6UK9ikM2kuwc625+PM5nicGLBbdVR8t5RenRAOyT+MSWNG36PmICIDeMu3klHHki6wZpwu18g3oVdDoCiMHz83Uw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=desiato.srs.infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=mv0z379R; arc=none smtp.client-ip=90.155.92.199 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=desiato.srs.infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="mv0z379R" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; 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=Z1Cr3VHZ2N5IQ8CUi2xKvTg3Y8erVu0IcZEsJQQswaY=; b=mv0z379RkB27IkO+P/y4veCLUs wp9AsKikdKINVJcE1S8BPGUn2fI6uHmEWzum4/qW95iFOv6EJr4qMS8vX3RDaXrcv2LP3k+1BzMuj T+oR3NTcG0lCqwgDssOvSOfUbj0E9ModlKR0OtLg5r+cSVGVrACBhLsyxOQXfDaDdVprSQgTApKX4 jvZDsIKOfcIXasCLrVePZrHSIutFc72rbhdNdU8BTvybdxHX2+8tB0PQz9u4QOvL1FAGwbJ84GNKQ UmUkWogpD43blH0UP4kI5fT2QF9I5Xis7FC2tHGXGKj+OzTzePU2IUg4oCi3gCys2mfGDKLoLUoWj wukrsG9A==; Received: from [2001:8b0:10b::4] (helo=u09cd745991455d.ant.amazon.com) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vjSV7-00000003DOr-3CGt; Sat, 24 Jan 2026 01:31:46 +0000 Message-ID: <41b50e06dab5cb6195b20016c372cdcaa56f1943.camel@infradead.org> Subject: Re: [PATCH 0/3] KVM: x86: Mitigate kvm-clock drift caused by masterclock update From: David Woodhouse To: Dongli Zhang , kvm@vger.kernel.org Cc: seanjc@google.com, pbonzini@redhat.com, paul@xen.org, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, linux-kernel@vger.kernel.org, joe.jin@oracle.com Date: Fri, 23 Jan 2026 17:31:40 -0800 In-Reply-To: <4ed7a646-707a-40c9-93f9-2289fedf5709@oracle.com> References: <20260115202256.119820-1-dongli.zhang@oracle.com> <285e30927dad5736df58aad5d957448e93b2d047.camel@infradead.org> <4ed7a646-707a-40c9-93f9-2289fedf5709@oracle.com> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-PmeCQUV37iEo8ytnwEMh" User-Agent: Evolution 3.52.3-0ubuntu1.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by desiato.infradead.org. See http://www.infradead.org/rpr.html --=-PmeCQUV37iEo8ytnwEMh Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2026-01-16 at 01:31 -0800, Dongli Zhang wrote: >=20 > With above changes in both QEMU and KVM, a same-host live migration of a = 4-vCPU > VM with approximately 10 seconds of downtime (introduced on purpose) resu= lts in > only about 4 nanoseconds of backward drift in my test environment. We may= even > be able to make more improvement from QEMU to rule out the remaining 4 na= noseconds. On the same host, even with TSC scaling, there is no excuse for *any* errors on live migration. The *offset* of the host =E2=86=92 guest TSC should remain precisely the sa= me. And the calculation of KVM clock from guest TSC should remain precisely the= same. Absolutely *zero* error. Don't bother with "improvements" which still don't get that right. --=-PmeCQUV37iEo8ytnwEMh 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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