From mboxrd@z Thu Jan 1 00:00:00 1970 From: Radim =?utf-8?B?S3LEjW3DocWZ?= Subject: Re: [PATCH] x86/kvm: fix condition to update kvm master clocks Date: Fri, 27 May 2016 20:11:40 +0200 Message-ID: <20160527181139.GA18797@potion> References: <1464274195-31296-1-git-send-email-rkagan@virtuozzo.com> <20160526201936.GA25334@potion> <20160527172809.GB17398@rkaganb.sw.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE To: Roman Kagan , kvm@vger.kernel.org, "Denis V. Lunev" , Owen Hofmann , Paolo Bonzini , Marcelo Tosatti Return-path: Received: from mx1.redhat.com ([209.132.183.28]:55531 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932737AbcE0SLo (ORCPT ); Fri, 27 May 2016 14:11:44 -0400 Content-Disposition: inline In-Reply-To: <20160527172809.GB17398@rkaganb.sw.ru> Sender: kvm-owner@vger.kernel.org List-ID: 2016-05-27 20:28+0300, Roman Kagan: > On Thu, May 26, 2016 at 10:19:36PM +0200, Radim Kr=C4=8Dm=C3=A1=C5=99= wrote: >> > atomic_read(&kvm_guest_has_master_clock) !=3D 0) >>=20 >> And I don't see why we don't want to enable master clock if the host >> switches back to TSC. >=20 > Agreed (even though I guess it's not very likely: AFAICS once switche= d > to a different clocksource, the host can switch back to TSC only upon > human manipulating /sys/devices/system/clocksource). Yeah, it's a corner case. Human would have to switch from tsc as well, automatic switch happens only when tsc is not useable anymore, AFAIK. >> > queue_work(system_long_wq, &pvclock_gtod_work); >>=20 >> Queueing unconditionally seems to be the correct thing to do. >=20 > The notifier is registered at kvm module init, so the work will be > scheduled even when there are no VMs at all. Good point, we don't want to call pvclock_gtod_notify in that case either. Registering (unregistering) with the first (last) VM should be good enough ... what about adding something based on this? diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 37af23052470..0779f0f01523 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -655,6 +655,8 @@ static struct kvm *kvm_create_vm(unsigned long type= ) goto out_err; =20 spin_lock(&kvm_lock); + if (list_empty(&kvm->vm_list)) + kvm_arch_create_first_vm(kvm); list_add(&kvm->vm_list, &vm_list); spin_unlock(&kvm_lock); =20 @@ -709,6 +711,8 @@ static void kvm_destroy_vm(struct kvm *kvm) kvm_arch_sync_events(kvm); spin_lock(&kvm_lock); list_del(&kvm->vm_list); + if (list_empty(&kvm->vm_list)) + kvm_arch_destroy_last_vm(kvm); spin_unlock(&kvm_lock); kvm_free_irq_routing(kvm); for (i =3D 0; i < KVM_NR_BUSES; i++) >> Interaction between kvm_gen_update_masterclock(), pvclock_gtod_work(= ), >> and NTP could be a problem: kvm_gen_update_masterclock() only has t= o >> run once per VM, but pvclock_gtod_work() calls it on every VCPU, so >> frequent NTP updates on bigger guests could kill performance. >=20 > Unfortunately, things are worse than that: this stuff is updated on > every *tick* on the timekeeping CPU, so, as long as you keep at least > one of your CPUs busy, the update rate can reach HZ. The frequency o= f > NTP updates is unimportant; it happens without NTP updates at all. >=20 > So I tend to agree that we're perhaps better off not fixing this bug = and > leaving the kvm_clocks to drift until we figure out how to do it with > acceptable overhead. Yuck ... the hunk below could help a bit. I haven't checked if the timekeeping code updates gtod and therefore sets 'was_set' even when the resulting time hasn't changed, so we might need to do more to avoid useless situations. diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index a8c7ca34ee5d..37ed0a342bf1 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -5802,12 +5802,15 @@ static DECLARE_WORK(pvclock_gtod_work, pvclock_= gtod_update_fn); /* * Notification about pvclock gtod data update. */ -static int pvclock_gtod_notify(struct notifier_block *nb, unsigned lon= g unused, +static int pvclock_gtod_notify(struct notifier_block *nb, unsigned lon= g was_set, void *priv) { struct pvclock_gtod_data *gtod =3D &pvclock_gtod_data; struct timekeeper *tk =3D priv; =20 + if (!was_set) + return 0; + update_pvclock_gtod(tk); =20 /* disable master clock if host does not trust, or does not