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 17F011FFC6E; Thu, 27 Mar 2025 08:10:50 +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=1743063054; cv=none; b=Eqp/eT2EyIGbhdLWr7jxb1Mv/GpzR3Rq0oaeuZ/VFEiry36ecUy1fib+tp7RdUKxjodCbOtI/5ACzCp+8rzdDvLSDUO2gRV/g8F6Q0PGj4TOaJOFzhpxMRz4YJhlvtgLsPgT9mAhU6v0ds0xA9//SDrCM6p8GbK4UlhjR7Y0w88= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743063054; c=relaxed/simple; bh=e5odK2PRBJXEv4YxSLBFs+Dv9VVjIh1oo4VKosx0RYc=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=pLkYuwC5zncstaH8yObhuW8nxOnpC95k08dX1/dufxUfAaR+KWek+ss64zVBxaO2h91HyIzbBODsX5gIMAFZtv/FJ+WnEhTzV2GuPA8M6cplpRal/48bPD+YwYnRTYKWU8ynFzC3XFGT8w4RDURIMUUJ3ZFWJbhMnZCNyPqIInI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (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=qkmx23NX; arc=none smtp.client-ip=90.155.92.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (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="qkmx23NX" 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=e5odK2PRBJXEv4YxSLBFs+Dv9VVjIh1oo4VKosx0RYc=; b=qkmx23NXv+iZDjEWUA9jsE5gxY lu5NZ45WkHcrQrNtd2kB6MPws1xQBjdxtNok9dSF5m/3091KImCyy1g5M4w35Dr1QdSb0GlargLSZ hP9LKwFWIvHN+aSJOY0x8Ffa7QtDG25BH2dSeGwRQaCvIr9GbvlkPEcxE4vbFQgqy5j+APKslp3La 7T4n4RMSG6KeC4+1B2pr9ATdVjaAYlxF7y/3JPIxZZKnG73VK6GnTfG1GB4wu6WvXLyZyfuabuOfx mE6WxkxkaJrpAK2wE4htbmRKJI6DUvQUFJbQqviO6gqzLioLfWznxaSew50AqznX33DGD81ic3/e2 Vt1i2VaQ==; Received: from [172.31.31.145] (helo=u09cd745991455d.lumleys.internal) by desiato.infradead.org with esmtpsa (Exim 4.98.1 #2 (Red Hat Linux)) id 1txiK7-00000005wfh-3moI; Thu, 27 Mar 2025 08:10:48 +0000 Message-ID: <830d2e06c064e24bd143650ce97522c2bc470a90.camel@infradead.org> Subject: Re: pvclock time drifting backward From: David Woodhouse To: Ming Lin Cc: Sean Christopherson , kvm@vger.kernel.org, Paolo Bonzini , linux-kernel@vger.kernel.org Date: Thu, 27 Mar 2025 08:10:47 +0000 In-Reply-To: References: <4eda127551d240b9e19c1eced16ad6f6ed5c2f80.camel@infradead.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-PWrEyYhwl7qjzJQn6i3Q" User-Agent: Evolution 3.52.3-0ubuntu1 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 --=-PWrEyYhwl7qjzJQn6i3Q Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, 2025-03-26 at 08:54 -0700, Ming Lin wrote: > I applied the patch series on top of 6.9 cleanly and tested it with my > debug tool patch. > But it seems the time drift still increased monotonically. >=20 > Would you help take a look if the tool patch makes sense? > https://github.com/minggr/linux/commit/5284a211b6bdc9f9041b669539558a6a85= 8e88d0 >=20 > The tool patch adds a KVM debugfs entry to trigger time calculations > and print the results. > See my first email for more detail. Your first message seemed to say that the problem occurred with live migration. This message says "the time drift still increased monotonically".=20 Trying to make sure I fully understand... the time drift between the host's CLOCK_MONOTONIC_RAW and the guest's kvmclock increases monotonically *but* the guest only observes the change when its master_kernel_ns/master_cycle_now are updated (e.g. on live migration) and its kvmclock is reset back to the host's CLOCK_MONOTONIC_RAW? Is this live migration from one VMM to another on the same host, so we don't have to worry about the accuracy of the TSC itself? The guest TSC remains consistent? And presumably your host does *have* a stable TSC, and the guest's test case really ought to be checking the PVCLOCK_TSC_STABLE_BIT to make sure of that? If all the above assumptions/interpretations of mine are true, I still think it's expected that your clock will jump on live migration *unless* you also taught your VMM to use the new KVM_[GS]ET_CLOCK_GUEST ioctls which were added in my patch series, specifically to preserve the mathematical relationship between guest TSC and kvmclock across a migration. --=-PWrEyYhwl7qjzJQn6i3Q Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCD9Aw ggSOMIIDdqADAgECAhAOmiw0ECVD4cWj5DqVrT9PMA0GCSqGSIb3DQEBCwUAMGUxCzAJBgNVBAYT AlVTMRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5jb20xJDAi BgNVBAMTG0RpZ2lDZXJ0IEFzc3VyZWQgSUQgUm9vdCBDQTAeFw0yNDAxMzAwMDAwMDBaFw0zMTEx MDkyMzU5NTlaMEExCzAJBgNVBAYTAkFVMRAwDgYDVQQKEwdWZXJva2V5MSAwHgYDVQQDExdWZXJv a2V5IFNlY3VyZSBFbWFpbCBHMjCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAMjvgLKj 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