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 DDC311C2AA; Sat, 9 May 2026 12:22:54 +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=1778329379; cv=none; b=cSe29V9WIZLm0cMI772GXhLkUo/urFNghAufJoAEEV5CK27YmsxFWRmIbBXdKFzYNlwWnn+Hu9ER9RDaGjuuQ7WIC0H+jt0aSBasEWpE5v9b+2rvqMNiHu2heSXzMO6RETjoqNmUjWPbEIDkHJTp14J7VEAd+Pi2YbKnYjH3r2U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778329379; c=relaxed/simple; bh=cqnOsjeFwLSK5/e3pRC4tKFhvZONJcUujM9cmmIOpNE=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=jjpoMD7uLtam4+vjnYmvCLGnFhnmzl8TB2cu8wBL4zCPGjfVecFsvRiV2x56xPElQ2Pzt0TZRrFJNVU8lICUlyQFc0GjPa20pvenIDo9bHrOhLEmZ8n0XAfGhpQLiljKzdS1+Z6+YLrhkDR9h4eGs1NqKK90/nELKLWjlT1g45E= 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=BZScMohW; 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="BZScMohW" 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=cqnOsjeFwLSK5/e3pRC4tKFhvZONJcUujM9cmmIOpNE=; b=BZScMohWrSgvzS1aF3b1qfRAdk J7XF1cLtmO+JiRpRNmXzT6yJKio5AevuOcuueag3wgEoofyL2xHRInXgDugx4BnDn6saFqyXjfEZK htdZOdhBEBzuXGBptJsTaafpawqElBPTuMasHAbpPadY7Poz/ka/DiHDzQZr5VwN0SKassyYn5WBN m7ILTgN0zdg4bmuw5Bk7R2UzdF+ABIPF4SJOsPMVW4vjAjca9/RDG0FGhXAPCPP48kN4+Rd4fLr9/ VdRgWj9foBrakyaivVJ5bSV/WdnV1pPYQ25k0xotpfvW2z7fhueib6AQH1mP2PgKvtLGOJPe2CUWX mtuhINJQ==; Received: from [172.31.31.148] (helo=u09cd745991455d.ant.amazon.com) by desiato.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wLghj-00000008OTU-2CS6; Sat, 09 May 2026 12:22:49 +0000 Message-ID: <16b69905d5d14a06dbb68f29355c58e2c7d2f0d2.camel@infradead.org> Subject: Re: [PATCH 3/3] KVM: x86: conditionally update masterclock data in pvclock_update_vm_gtod_copy() 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: Sat, 09 May 2026 13:22:47 +0100 In-Reply-To: <20260115202256.119820-4-dongli.zhang@oracle.com> References: <20260115202256.119820-1-dongli.zhang@oracle.com> <20260115202256.119820-4-dongli.zhang@oracle.com> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-/xYCiYvVYoFjCKHLvQQF" 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 --=-/xYCiYvVYoFjCKHLvQQF Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2026-01-15 at 12:22 -0800, Dongli Zhang wrote: > The pvclock_update_vm_gtod_copy() function always unconditionally updates > ka->master_kernel_ns and ka->master_cycle_now whenever a > KVM_REQ_MASTERCLOCK_UPDATE occurs. Unfortunately, each masterclock update > increases the risk of kvm-clock drift. >=20 > If pvclock_update_vm_gtod_copy() is not called from > vcpu_enter_guest()-->kvm_update_masterclock(), we keep the existing > workflow. The argument 'forced' is introduced to tell where it is from. >=20 > Otherwise, we avoid updating the masterclock if it is already > active and will remain active. In such cases, updating the masterclock > data is not beneficial and can instead lead to kvm-clock drift. >=20 > As a result, this patch minimizes the chance of unnecessary masterclock > data updates to avoid kvm-clock drift. >=20 > Cc: David Woodhouse > Signed-off-by: Dongli Zhang Hmm... so the only caller of pvclock_update_vm_gtod_copy() that doesn't set the 'force' argument is the one in kvm_update_masterclock(), so we are asserting that kvm_update_masterclock() never needs to *change* the masterclock origin point, if it was already set? The gtod notifier callback in pvclock_gtod_update_fn() also ends up setting KVM_REQ_MASTERCLOCK_UPDATE, and is triggered by an actual host timekeeping update (which could potentially be from a clocksource change). It also hypothetically possible that the clocksource changes from TSC =E2=86=92 HPET =E2=86=92 TSC, switching back to TSC again before t= he masterclock update ever gets to run. Or maybe a suspend/resume? 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