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 C3874377009; Wed, 27 May 2026 08:43:07 +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=1779871391; cv=none; b=IjeJpEgqKwIlrWy1/kHderapiICnw8Y3eZxCasozd6aw7GTdxqeDIBkV9PcWlSjHhfaRFM5BCIEloSfbhurAC6DJ/v17NkDaI0O2hnja3FUlt6xVTUU5a1Hrp6vzi78UFsdFveXLoK7nyYhqmNdXSiUC+vH/aGg6UCPVmOTDs8M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779871391; c=relaxed/simple; bh=mh6dUGAyQ1ZOqNwASivtSZC/xgcR9rPO3v/VTvhWmgo=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=gCsn840Akl40hSA+fg9+xI3thFVzdoCwIilK0NK6J9HTycGj5/wh6vgaCUZmZVTnFUxEAQMaqu2E/pvjMm0kqm7tCAVG2ajfLj8TGPRuKODFtu/nUJ1okHzJUoWA6CiSOgrwyBj35N1VBUhbhshdB4V4GhdJ5gpaq7eifQSeb8E= 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=k1eHR8Tv; 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="k1eHR8Tv" 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=mh6dUGAyQ1ZOqNwASivtSZC/xgcR9rPO3v/VTvhWmgo=; b=k1eHR8TvRkOvLA6D7qy9kOIS0t /wnweZhdzmMd8ZR64WdpA5gfWhDXQfJZgxx5KWFxEt1Hg4tLfdFvFBeJpCdCZxG4dKXDAEim28cSk n2onG8xItWGX9G3f1iTTaeU8fdFpzZGojt07lMAtNI5L5gJbyPVTLwUVlPU8681lybVvRJHZK7Su/ 3+MpOm+9TAEVoARz2T513fc2k4C+XaR/HotZ4yWQQtQ2pKjkQUOCUohT7A1qtzMOOwCOIa0+5CKS4 qB9MaVOMawKNeSvzt5Yk+su941eJfdcTV921050N4O/zRHGA6wDNCEvAJQxImi72JHqSZij1d+nxH FPf0DcNw==; Received: from 54-240-197-231.amazon.com ([54.240.197.231] helo=lhr50-en-acc-sw5.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wS9ql-000000027sF-2y5v; Wed, 27 May 2026 08:42:52 +0000 Message-ID: Subject: Re: [RFC] KVM/x86: Killing kvm_get_time_and_clockread() in favour of ktime_get_snapshot() From: David Woodhouse To: Vitaly Kuznetsov , Sean Christopherson , Paolo Bonzini , Thomas Gleixner , John Stultz , Michael Kelley Cc: Marcelo Tosatti , "Christopher S. Hall" , Stephen Boyd , Miroslav Lichvar , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , "K. Y. Srinivasan" , Haiyang Zhang , Wei Liu , Dexuan Cui , Daniel Lezcano , kvm@vger.kernel.org, linux-hyperv@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org Date: Wed, 27 May 2026 09:42:49 +0100 In-Reply-To: <87zf1ljluq.fsf@redhat.com> References: <87zf1ljluq.fsf@redhat.com> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-Uw10D9j7ylHvFV4A19qe" 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 --=-Uw10D9j7ylHvFV4A19qe Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, 2026-05-27 at 10:30 +0200, Vitaly Kuznetsov wrote: > David Woodhouse writes: >=20 > ... >=20 > >=20 > > Then in 2018, Vitaly Kuznetsov added Hyper-V TSC page support in > > commit b0c39dc68e3b ("x86/kvm: Pass stable clocksource to guests when > > running nested on Hyper-V"), which extended vgettsc() to handle the > > HVCLOCK case. > >=20 > > I'd quite like to kill it all with fire and make KVM use > > ktime_get_snapshot() instead. >=20 > The main motivation is reducing the complexity of KVM's timekeeping code > I guess? Reduce the complexity, and clean it up to use the core kernel snapshot facility as $DEITY intended, yes. > The statement "TSC isn't reliable" deserves a book of its own :-) ... deserves a whole wing in a rehab facility... Thanks for the clarification. > > One simple option that occurs to me would be to add a 'cycles_raw' > > value to the system_time_snapshot, for PV clocksources like hyperv and > > kvmclock to populate with the original TSC reading. >=20 > Personally, I don't see this as such an ugly hack. Yeah, having thrown that together last night to see what it looks like, I'm not all that unhappy with it. And it does indeed let me provide precise vmclock paired time when the clocksource is kvmclock, too. There are probably some details to tweak, and I'm sure Thomas will call me a moron again, but I think it's worth it for the resulting cleanup :) --=-Uw10D9j7ylHvFV4A19qe 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 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