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 35F853101B6 for ; Thu, 21 May 2026 21:06:51 +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=1779397612; cv=none; b=A4LMFss/iJlz8iO0bB3Qoq3Zw2KtzM/DDfqAFvifzAn7h58yCD8Rmqne7IDHTzD3q0/5vlEMXoif0EDo6hmsLvSg1C8of76ymRE7Sl/EsKM1LdMTjpKeVPuoEVMNZrnJEdVFcoiBv2KO2PFAKGRaidK1C9mqjHeUD7juDUL9GLQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779397612; c=relaxed/simple; bh=mKPf1kArN5+UgvyP/NlBXLKP7I433+zDsrNP4yTLbzk=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=M6mCpSNv3xdjcZIoczoo654eI7nXhXSAygXo3obfqhphyEpvbdA3NiuQpqCUjuyKWesHnX9i2qSG8LIbhz2sZu4nVpyAgOpUJClqGjFE4KKALSIepVmI+p2UxYGakCH9JXOsIT+SANhLHKyHcbEIlXWuTtqs+nOMg3aNW3HpqHQ= 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=CeAPcFAp; 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="CeAPcFAp" 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=mKPf1kArN5+UgvyP/NlBXLKP7I433+zDsrNP4yTLbzk=; b=CeAPcFApljL+wIiP8xyY59pM1h br4iMQafBvNeqLNizLfxV18PSk2me8ksicDltcKD2ZJNiDXwYk4A1RKa/amg6iKeOsA+WA1J93Kj4 PrePm9dxATAwhyP+Tqy6LIkGxHLu0L6WJpLRs1nur8sEjAxxD4evUcLRYgzl2cS7yYnA/4wtb/x33 PvKOxp7JfC7QvQYanMBBikNNOj6Cg5P327nQmBwwBHB2ifyC5mT1zwSFDNLe6x7NediG6MHPUuLRH GM9IrplceUY4np7UdnVVg/FJJmpCY9Uam5Pt8qUksGGCUvnq7qpE92JY5oRXDwVD7f3YWBFonZ9og Mz2ensnA==; 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 1wQAbG-000000091qg-31XG; Thu, 21 May 2026 21:06:39 +0000 Message-ID: <6c7137748ccea288fdf55e7c2e24817ccc1673ef.camel@infradead.org> Subject: Re: [RFC PATCH v2 0/8] timekeeping: Fix draft tracking precision and add feed-forward discipline via vmclock From: David Woodhouse To: Thomas Gleixner , Miroslav Lichvar Cc: Richard Cochran , Wen Gu , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , John Stultz , Stephen Boyd , Anna-Maria Behnsen , Frederic Weisbecker , Shuah Khan , Peter Zijlstra , Thomas =?ISO-8859-1?Q?Wei=DFschuh?= , Arnd Bergmann , Julien Ridoux , Ryan Luu , linux-kernel@vger.kernel.org, Marcelo Tosatti Date: Thu, 21 May 2026 22:06:37 +0100 In-Reply-To: <87o6i8vcni.ffs@tglx> References: <20260517220326.4625-1-dwmw2@infradead.org> <0d32da75fa88c92ac0225ef23a9045afdf2ac9fe.camel@infradead.org> <87o6i8vcni.ffs@tglx> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-PO41sy5CQkXT5EVn30ZJ" 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 casper.infradead.org. See http://www.infradead.org/rpr.html --=-PO41sy5CQkXT5EVn30ZJ Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2026-05-21 at 20:30 +0200, Thomas Gleixner wrote: >=20 > As I said in the other thread, that's just creating yet another private > mechanism instead of collecting the counter value together with e.g. > CLOCK_REALTIME=C2=A0 On the plus side, at least he wasn't providing a counter value at *all* for the system timestamps, which is better than using a bogus one :) Can we have a signed-off-by for your ktime_get_snapshot_id() please? > or utilizing the PMT correlated one which is available in > get_device_crosstime_stamp(). AFAICT that was the *only* one he was exposing, wasn't it? The vmclock driver literally did expose the cycle count used to create the device timestamp, which is equivalent to PTM and looked correct for that part? > Can we please stop creating specialized interfaces and instead make them > generic, so they can be used for everything? Of course. > Then you can go and extend the posix-timer interface with > clock_set_time_reference() (or whatever name we come up with) and > provide the functionality for all steerable clocks. That'd allow chronyd > to completely ignore the kernel side NTP PLL and do everything in user > space. That obviously needs some thought and input from the chrony > folks, but that's a long term useful solution and not some 'scratch my > itch' side channel. Yeah, that's a neat idea. I deliberately hadn't even *proposed* a userspace API for that at all yet; for the timekeeping part I'm just working on the basic *concepts* and the accounting fixes that make it all actually work, with a hack to unconditionally call it directly from vmclock for now. In order to solicit exactly that feedback and design a long term solution that works for everyone, before going too far down any particular implementation path. I like clock_set_time_reference(). I'll have a play and see what I can come up with. It would want to carry error bounds information too. Having a clock_get_time_reference() would be nice too for QEMU to use, but that would just be a snapshot and wouldn't get updated when the clock is adjusted. While the /dev/vmclock_host thing I have in my tree right now can at least use a gtod notifier and the userspace device is pollable. And it can export everything we need in one go. More thought required on that one... but I'm very keen *not* to let that one get forgotten, because I want this to work optimally for the general case of QEMU running on a standard general purpose host, not *only* the dedicated hosting setup where userspace is prepared to do all the work. --=-PO41sy5CQkXT5EVn30ZJ 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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