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 B296A3E3C48; Fri, 26 Jun 2026 13:04:27 +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=1782479070; cv=none; b=XcqXjXE73wcccJXwIbo34onaGILbhUyKHqVsqvsPFKP2lRLHKweO8n4gP8FSTJlHO+frvlnFiobVXgBL9fZyq+yqIiSkbOOvcrKQIoGvRG1J0GJiI8FbnQoBn5682LEaHfZfhDFfLCAMOtCqYo5JmBlrGRierxgOXFINfMExjzc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782479070; c=relaxed/simple; bh=uzKGErtMmxktxmF9Q5RDlkms6wcbY6dIab3o0yeQRr4=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=nZzC2lCgk4KEuqEyRQ6junD1sQGF645/Ipa7s5TxvS23tIxN22rQyz+KAA7KKiwC8QfVxkKy4hH/WByXVc8mAEaGxe0WTgap+b/i9r39FkWJw3ipWkYTGyipnw+g81CgULoTuETUBgsMjcHPo00PkUL3VPvt/JQ7yalb1c3MYTI= 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=aa3NFxlu; 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="aa3NFxlu" 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=p7aRzIif6Ecb7cZno4wzYdFgvJT2Wn5O7xaduJZCRAU=; b=aa3NFxluyzZQ5OB0iK+U4uRxf9 eqQof+klGLqqTaVLBSEZQxAYAnxR5I+POsB8l41/nHl9h3LwjSBcxD6/hgta8IZM5aXWfNpZKrk74 iOER01TVjSH26NjzjYaRwFBt870XTf5hTK8B9mCNj1+QWOOIspLhgRHd8KOvIs0kiFfJls6S76Kzx ZWqVI/MkbOpGWl4xV8hg4NgjsaRATxqmsICO/fCVDtWqpdHVziptHPf6czDLckPKgrkZh6yHpKnDm 99b1mxMCk3LVjjFZsdIex0c5ALIr05PIG9NFSprv9nQHOLlUfOJMQSm1umuhcEv3fLVgiic764vWE tOJRfEGg==; Received: from [2001:8b0:10b:5:f3d6:3c0c:6c61:8d10] (helo=u09cd745991455d.ant.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wd6EJ-0000000Br9l-3YRu; Fri, 26 Jun 2026 13:04:24 +0000 Message-ID: Subject: Re: [PATCH v2] KVM: x86: Rate-limit global clock updates on vCPU load From: David Woodhouse To: Lei Chen , seanjc@google.com Cc: igor@gooddata.com, jan.cipa@gooddata.com, jaroslav.pulchart@gooddata.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, pbonzini@redhat.com Date: Fri, 26 Jun 2026 14:04:23 +0100 In-Reply-To: <20260409142226.2581-1-lei.chen@smartx.com> References: <20260409142226.2581-1-lei.chen@smartx.com> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-Wus06MMlFQG2jRGofuUt" User-Agent: Evolution 3.60.3-0ubuntu1~ppa6 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 --=-Wus06MMlFQG2jRGofuUt Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2026-04-09 at 22:22 +0800, Lei Chen wrote: >=20 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -5210,8 +5210,13 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int= cpu) > =C2=A0 * On a host with synchronized TSC, there is no need to update > =C2=A0 * kvmclock on vcpu->cpu migration > =C2=A0 */ > - if (!vcpu->kvm->arch.use_master_clock || vcpu->cpu =3D=3D -1) > - kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu); > + if (!vcpu->kvm->arch.use_master_clock || vcpu->cpu =3D=3D -1) { > + if (__ratelimit(&vcpu->kvm->arch.kvmclock_update_rs)) > + kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu); > + else > + kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu); > + } > + > =C2=A0 if (vcpu->cpu !=3D cpu) > =C2=A0 kvm_make_request(KVM_REQ_MIGRATE_TIMER, vcpu); > =C2=A0 vcpu->cpu =3D cpu; I don't like this. We do the GLOBAL_CLOCK_UPDATE in non-masterclock mode because theoretically, a *failure* to do so could cause the guest to observe non-monotonicity across vCPUs. With this rate-limiting, we *sometimes* protect against that non- monotonicity. Why even bother? Why not just rip it out completely, in that case? AIUI the guest should cope with anyway if its PVCLOCK_TSC_STABLE_BIT isn't set, and enforce monotonicity for itself side in __pvclock_clocksource_read() ?=20 But also, I would *love* to kill non-masterclock mode completely. In this nested case, the L1 TSCs *are* actually in sync; it just don't have the +invtsc CPUID to *promise* that they will be. Could we find a way for L1 KVM to use masterclock mode for its nested L2 VMs even in this case? Could we make it use PVCLOCK_TSC_STABLE_BIT to enable masterclock mode, instead? At at the very least, could we find a way for L1 to trigger the GLOBAL_CLOCK_UPDATE only if an *actual* discrepancy in its TSCs is found? 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