From patchwork Sun Apr 7 13:15:42 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: David Woodhouse X-Patchwork-Id: 13620191 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 C247C38F91 for ; Sun, 7 Apr 2024 13:15:52 +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=1712495759; cv=none; b=jafhLNL+JQ0eZMjD+xLnLFYmGo/lBRXaRs93JgtLzonUJsMYu375tnRHnphN6fOfvjGJAFC+NOM8W1B1AGTmXv7yoTS1Zy0+lMqihW/SMG/j+Cvl24Yd+GUVTAgYkzyFsgLdZhbweXRSSk3iLzyIgB6nakYhEipMt6lDsdm4Guk= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712495759; c=relaxed/simple; bh=W//Kpb2brgWrc1wNHAULEHjm1ufICrdfsdBTR7lKqMY=; h=Message-ID:Subject:From:To:Cc:Date:Content-Type:MIME-Version; b=KadBvXU3v/XXmEqsI27tqLO+lHYhWKQ1FX8TUZg7rY/jQxpRxZUUyaqpKI49pNWXZZSUj2n9XzKGRIm+VPoAVOOC1xWCzFFilb65dJuKck51efw834B4WFAV8dUBZkw6b5b77pVxmZpl7EYcf8K7THZNfiNAIVtHk+/yfcnP76w= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=none (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=Vcma9UbT; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (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="Vcma9UbT" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=MIME-Version:Content-Type:Date:Cc:To: From:Subject:Message-ID:Sender:Reply-To:Content-Transfer-Encoding:Content-ID: Content-Description:In-Reply-To:References; bh=JpFyhs5kbf53e6EBHrtkWnyzb+nn5cSKhdz4N1pQoH0=; b=Vcma9UbT5hQWSe8vtpHAduSKwf GVKTZ8iMwkyzxyZEbnqrUXtJccUsxemOVcUMrJvlgWHdWtUf9DUMR/5zyiNutaa2AQUVkWVwm3gRl BsoOAWA8+MtR3aEKGmnP+ZCxTvPJjeMTmIWaDdX6U5EpNlXDdnsn6U3hZvzhRAcR+q4XccGolpg0b Xwg274ewReXhaMG8qMdarBhBKB9mg6tG9mJNXcs8m32uzzeab3MphAsrp1+k2TbWqIqsuJ6RefFg/ iwKFzrBz/c875DpX8pW22C0f5N2NIwD0VDsOJBqAaC+UFm/7pfZbKRiJgdcH//4NW71DfXL3ppZFA aJZaQl3Q==; Received: from [2001:8b0:10b:5:9405:2cc9:dabd:eacb] (helo=u3832b3a9db3152.ant.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.97.1 #2 (Red Hat Linux)) id 1rtSN5-0000000FCgj-0MTz; Sun, 07 Apr 2024 13:15:43 +0000 Message-ID: <7e0040f70c629d365e80d13b339a95e0affa6d61.camel@infradead.org> Subject: [PATCH] KVM: x86/xen: Do not corrupt KVM clock in kvm_xen_shared_info_init() From: David Woodhouse To: kvm@vger.kernel.org Cc: pbonzini@redhat.com, Sean Christopherson , Jim Mattson , Simon Veith , Jack Allister , paul , Joao Martins Date: Sun, 07 Apr 2024 14:15:42 +0100 User-Agent: Evolution 3.44.4-0ubuntu2 Precedence: bulk X-Mailing-List: kvm@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 From: David Woodhouse The KVM clock is an interesting thing. It is defined as "nanoseconds since the guest was created", but in practice it runs at two *different* rates — or three different rates, if you count implementation bugs. Definition A is that it runs synchronously with the CLOCK_MONOTONIC_RAW of the host, with a delta of kvm->arch.kvmclock_offset. But that version doesn't actually get used in the common case, where the host has a reliable TSC and the guest TSCs are all running at the same rate and in sync with each other, and kvm->arch.use_master_clock is set. In that common case, definition B is used: There is a reference point in time at kvm->arch.master_kernel_ns (again a CLOCK_MONOTONIC_RAW time), and a corresponding host TSC value kvm->arch.master_cycle_now. This fixed point in time is converted to guest units (the time offset by kvmclock_offset and the TSC Value scaled and offset to be a guest TSC value) and advertised to the guest in the pvclock structure. While in this 'use_master_clock' mode, the fixed point in time never needs to be changed, and the clock runs precisely in time with the guest TSC, at the rate advertised in the pvclock structure. The third definition C is implemented in kvm_get_wall_clock_epoch() and __get_kvmclock(), using the master_cycle_now and master_kernel_ns fields but converting the *host* TSC cycles directly to a value in nanoseconds instead of scaling via the guest TSC. One might naïvely think that all three definitions are identical, since CLOCK_MONOTONIC_RAW is not skewed by NTP frequency corrections; all three are just the result of counting the host TSC at a known frequency, or the scaled guest TSC at a known precise fraction of the host's frequency. The problem is with arithmetic precision, and the way that frequency scaling is done in a division-free way by multiplying by a scale factor, then shifting right. In practice, all three ways of calculating the KVM clock will suffer a systemic drift from each other. Definition C should simply be eliminated. Commit 451a707813ae ("KVM: x86/xen: improve accuracy of Xen timers") worked around it for the specific case of Xen timers, which are defined in terms of the KVM clock and suffered from a continually increasing error in timer expiry times. Definitions A and B do need to coexist, the former to handle the case where the host or guest TSC is suboptimally configured. But KVM should be more careful about switching between them, and the discontinuity in guest time which could result. In particular, KVM_REQ_MASTERCLOCK_UPDATE will take a new snapshot of time as the reference in master_kernel_ns and master_cycle_now, yanking the guest's clock back to match definition A at that moment. There is no need to do such an update when a Xen guest populates the shared_info page. This seems to have been a hangover from the very first implementation of shared_info which automatically populated the vcpu_info structures at their default locations, but even then it should just have raised KVM_REQ_CLOCK_UPDATE on each vCPU instead of using KVM_REQ_MASTERCLOCK_UPDATE. And now that userspace is expected to explicitly set the vcpu_info even in its default locations, there's not even any need for that either. Fixes: 629b5348841a1 ("KVM: x86/xen: update wallclock region") Signed-off-by: David Woodhouse Reviewed-by: Paul Durrant --- arch/x86/kvm/xen.c | 2 -- 1 file changed, 2 deletions(-) diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c index f65b35a05d91..5a83a8154b79 100644 --- a/arch/x86/kvm/xen.c +++ b/arch/x86/kvm/xen.c @@ -98,8 +98,6 @@ static int kvm_xen_shared_info_init(struct kvm *kvm) wc->version = wc_version + 1; read_unlock_irq(&gpc->lock); - kvm_make_all_cpus_request(kvm, KVM_REQ_MASTERCLOCK_UPDATE); - out: srcu_read_unlock(&kvm->srcu, idx); return ret;