From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 74CF128EA for ; Sat, 4 May 2024 07:44:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1714808655; cv=none; b=ETtNib6nqZqYpDfZ1XXcc4Ac9IXLUnRVMIyAkLBfcjqkFGTad9N7jprARrAvcnEwbL57k84j/ccBdoxqZ63XLxfW997u5t9n5pJtzfcoCREAo5vAhxd3ufkaWVbLFjw2xt/lFqgfUuMNh8FXuUWztGUAp97O+GGUl01fmYnBT/4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1714808655; c=relaxed/simple; bh=+CN3pnC6QWhowtu4+MVaqdx3fIV2fqPHQVc21m9r/bQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=YJcfpiTYCCLmZKlX7ZTBsCG1i9tvvdHXUWwCHsenB/1nVkuP0m7eALmW04Oz0fAzBXwq8Nr6TmzwlCNwug2wgrh8TLYbylvrbGz2sCL56GBoYlQ82z8p/RwJkILBa9xiLs/4WRzuw16ZPMkxpKqNQnI1/RUAzj9veVsu0j9R1SQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=djmhxBhM; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="djmhxBhM" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1714808652; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=xU5umL+P+Q+gcA8Y/3yX0AYaEr9mECnJdAoU1QaQSxw=; b=djmhxBhMZ69xtOQt1xJQf+PTvSammi8wW5rHzecKhRGkItjzypMJL8YVPcjK1hcSesjkAO iH4I439bfNxBCpRHgsSJ15XWLFELkE4nDCEzx6YF4aBQVybrH5J8Ad68jAju8G8XfU5aCl oauKbkGKajhHULBl1ueVDBKfYbzG/cw= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-675-mQE3_v7uMwijUuGdmrozOg-1; Sat, 04 May 2024 03:44:09 -0400 X-MC-Unique: mQE3_v7uMwijUuGdmrozOg-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 053D11005055; Sat, 4 May 2024 07:44:08 +0000 (UTC) Received: from tpad.localdomain (unknown [10.96.133.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1FA31AC6B; Sat, 4 May 2024 07:44:07 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 5469F400DF404; Sat, 4 May 2024 04:42:47 -0300 (-03) Date: Sat, 4 May 2024 04:42:47 -0300 From: Marcelo Tosatti To: David Woodhouse Cc: kvm@vger.kernel.org, Paolo Bonzini , Jonathan Corbet , Sean Christopherson , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Paul Durrant , Shuah Khan , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Oliver Upton , jalliste@amazon.co.uk, sveith@amazon.de, zide.chen@intel.com, Dongli Zhang Subject: Re: [PATCH v2 01/15] KVM: x86/xen: Do not corrupt KVM clock in kvm_xen_shared_info_init() Message-ID: References: <20240427111929.9600-1-dwmw2@infradead.org> <20240427111929.9600-2-dwmw2@infradead.org> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20240427111929.9600-2-dwmw2@infradead.org> X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.1 On Sat, Apr 27, 2024 at 12:04:58PM +0100, David Woodhouse wrote: > 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. > > Eventually, definition C should just 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. That commit notes that get_kvmclock_ns() is non-trivial to fix > and says "I'll come back to that", which remains true. > > 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. KVM_REQ_MASTERCLOCK_UPDATE stops the vcpus because: * To avoid that problem, do not allow visibility of distinct * system_timestamp/tsc_timestamp values simultaneously: use a master * copy of host monotonic time values. Update that master copy * in lockstep. > When invoked from in 'use_master_clock' mode, kvm_update_masterclock() > should probably *adjust* kvm->arch.kvmclock_offset to account for the > drift, instead of yanking the clock back to defintion A. You are likely correct... > But in the meantime there are a bunch of places where it just doesn't need to be > invoked at all. > > To start with: 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; > -- > 2.44.0 So KVM_REQ_MASTERCLOCK_UPDATE is to avoid the race above. In what contexes is kvm_xen_shared_info_init called from again? Not clear to me KVM_REQ_MASTERCLOCK_UPDATE is not needed (or that is needed, for that matter...).