From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7C487CAC592 for ; Mon, 22 Sep 2025 16:59:36 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1v0jsX-0003K7-Df; Mon, 22 Sep 2025 12:59:05 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1v0jsU-0003Jh-45 for qemu-devel@nongnu.org; Mon, 22 Sep 2025 12:59:02 -0400 Received: from casper.infradead.org ([2001:8b0:10b:1236::1]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1v0jsQ-0005Zt-Jj for qemu-devel@nongnu.org; Mon, 22 Sep 2025 12:59:01 -0400 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=AZUL/dI+nZtipFCFOlfWl0vPVQjXQZsbUVJwbAH7R68=; b=ocVFhNbraqNLIC/1NkTlqCiDMF jnYhlZD4eoeYH92twoLVH1o9dnYhZ2ElyxP58Bq39MfIlkBYUULIjewHVruXo4Sk3tiX0BLMvAAT7 zst1SAscGK0ltJdnkWUTN6EECtKvJelmeKwsdOeqbJtgph+sxf7zwOOigYwV+Z0enuaQPWOIJmDYN mRDazvNo+5OLZYGdzX3FGAblq959izxRHNU6WnEYHJWXEYymZVSqvulLF2su38WxDDMn2XQTlZjME 5y7/CivD2uU0tgrnVQ+xBfafj9G2rVChiCvVuGbkiOj+jc/mfHMt6X+NXI6kddZPLK7Q3ZXg/TgOM zL4Ll1/g==; Received: from 54-240-197-238.amazon.com ([54.240.197.238] helo=freeip.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1v0jsI-0000000BidW-3YwU; Mon, 22 Sep 2025 16:58:50 +0000 Message-ID: <3d30b662b8cdb2d25d9b4c5bae98af1c45fac306.camel@infradead.org> Subject: Re: Should QEMU (accel=kvm) kvm-clock/guest_tsc stop counting during downtime blackout? From: David Woodhouse To: Dongli Zhang , qemu-devel@nongnu.org Cc: kvm@vger.kernel.org Date: Mon, 22 Sep 2025 17:58:50 +0100 In-Reply-To: <2d375ec3-a071-4ae3-b03a-05a823c48016@oracle.com> References: <2d375ec3-a071-4ae3-b03a-05a823c48016@oracle.com> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-a2WRPj6JjCuR+q8i2m5D" User-Agent: Evolution 3.52.3-0ubuntu1 MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org. See http://www.infradead.org/rpr.html Received-SPF: none client-ip=2001:8b0:10b:1236::1; envelope-from=BATV+653b648b0a91ad8bd62e+8065+infradead.org+dwmw2@casper.srs.infradead.org; helo=casper.infradead.org X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, SPF_HELO_NONE=0.001, SPF_NONE=0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --=-a2WRPj6JjCuR+q8i2m5D Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2025-09-22 at 09:37 -0700, Dongli Zhang wrote: > Hi, >=20 > Would you mind helping confirm if kvm-clock/guest_tsc should stop countin= g > elapsed time during downtime blackout? >=20 > 1. guest_clock=3DT1, realtime=3DR1. > 2. (qemu) stop > 3. Wait for several seconds. > 4. (qemu) cont > 5. guest_clock=3DT2, realtime=3DR2. >=20 > Should (T1 =3D=3D T2), or (R2 - R1 =3D=3D T2 - T1)? Neither. Realtime is something completely different and runs at a different rate to the monotonic clock. In fact its rate compared to the monotonic clock (and the TSC) is *variable* as NTP guides it. In your example of stopping and continuing on the *same* host, the guest TSC *offset* from the host's TSC should remain the same. And the *precise* mathematical relationship that KVM advertises to the guest as "how to turn a TSC value into nanoseconds since boot" should also remain precisely the same. KVM already lets you restore the TSC correctly. To restore KVM clock correctly, you want something like KVM_SET_CLOCK_GUEST from https://lore.kernel.org/all/20240522001817.619072-4-dwmw2@infradead.org/ For cross machine migration, you *do* need to use a realtime clock reference as that's the best you have (make sure you use TAI not UTC and don't get affected by leap seconds or smearing). Use that to restore the *TSC* as well as you can to make it appear to have kept running consistently. And then KVM_SET_CLOCK_GUEST just as you would on the same host. 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