From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [qemu patch 2/2] kvmclock: reduce kvmclock difference on migration Date: Mon, 14 Nov 2016 16:15:21 -0200 Message-ID: <20161114181518.GA14076@amt.cnet> References: <20161114123700.158592605@redhat.com> <20161114140028.GA25935@amt.cnet> <62d634ab-70ad-4be7-1622-f2e3a9d865fe@redhat.com> <20161114145054.GA28663@amt.cnet> <67bffd95-2e4e-7273-c154-a3fdfe622387@redhat.com> <20161114154015.GA30048@amt.cnet> <20161114171318.GA6336@amt.cnet> <14044cda-054d-94eb-8d91-7ad3a1e0869e@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: kvm@vger.kernel.org, qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , Juan Quintela , Radim Krcmar , Eduardo Habkost To: Paolo Bonzini Return-path: Received: from mx1.redhat.com ([209.132.183.28]:40608 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932899AbcKNSPi (ORCPT ); Mon, 14 Nov 2016 13:15:38 -0500 Content-Disposition: inline In-Reply-To: <14044cda-054d-94eb-8d91-7ad3a1e0869e@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Mon, Nov 14, 2016 at 06:20:29PM +0100, Paolo Bonzini wrote: > > > On 14/11/2016 18:13, Marcelo Tosatti wrote: > > On Mon, Nov 14, 2016 at 05:43:33PM +0100, Paolo Bonzini wrote: > >> > >> > >> On 14/11/2016 16:40, Marcelo Tosatti wrote: > >>> static bool kvmclock_src_use_reliable_get_clock(void *opaque) > >>> { > >>> KVMClockState *s = opaque; > >>> > >>> /* > >>> * On machine types that support reliable KVM_GET_CLOCK, > >>> * if host kernel does provide reliable KVM_GET_CLOCK, > >>> * set src_use_reliable_get_clock=true so that destination > >>> * avoids reading kvmclock from memory. > >>> */ > >>> if (s->mach_use_reliable_get_clock && kvm_has_adjust_clock_stable()) > >>> { > >>> s->src_use_reliable_get_clock = true; > >>> } > >>> > >>> return s->mach_use_reliable_get_clock; > >>> } > >>> > >>> > >>> Ah, OK, done. > >> > >> s->src_use_reliable_get_clock should not be set with > >> KVM_CHECK_EXTENSION, but rather from the flags returned by KVM_GET_CLOCK. > > > > Well, thats not right: What matters is the presence of get_kvmclock_ns > > which returns a value that the guest sees. > > > > get_kernel_monotonic_clock() + kvmclock_offset + > > (rdtsc() - tsc_timestamp) > > > > IOW what the guest sees. And you changed that in > > > > commit 108b249c453dd7132599ab6dc7e435a7036c193f > > Author: Paolo Bonzini > > Date: Thu Sep 1 14:21:03 2016 +0200 > > > > KVM: x86: introduce get_kvmclock_ns > > > > And the correct behaviour (once KVM_GET_CLOCK is fixed per > > previous message to return rdtsc - tsc_timestamp for the > > non masterclock case) depends on this commit above, > > not on masterclock. > > This commit in turn only gets the correct behavior if > "vcpu->arch.hv_clock.flags & PVCLOCK_TSC_STABLE_BIT" (and it will be > changed soon to ka->use_masterclock). KVM_CHECK_EXTENSION can still > return KVM_CLOCK_TSC_STABLE even if the masterclock is disabled, > because KVM_CHECK_EXTENSION only tells you which flags are known for > this version of the KVM module. What QEMU wants is to use KVM_GET_CLOCK at pre_save independently of whether masterclock is enabled or not... it just depends on KVM_GET_CLOCK being correct for the masterclock case (108b249c453dd7132599ab6dc7e435a7036c193f). So a "reliable KVM_GET_CLOCK" (that does not timebackward when masterclock is enabled) is much simpler to userspace than "whether masterclock is enabled or not". If you have a reason why that should not be the case, let me know. > To see if the masterclock is enabled _now_, you need to check what > KVM_GET_CLOCK sets in the flags. From the KVM_CLOCK_TSC_STABLE patch: > > user_ns.flags = kvm->arch.use_master_clock ? KVM_CLOCK_TSC_STABLE : 0; Again, whether masterclock is enable is independent of being able to use KVM_GET_CLOCK at pre_save.