From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751008AbdAPRJC (ORCPT ); Mon, 16 Jan 2017 12:09:02 -0500 Received: from mx1.redhat.com ([209.132.183.28]:51822 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750728AbdAPRJA (ORCPT ); Mon, 16 Jan 2017 12:09:00 -0500 Date: Mon, 16 Jan 2017 15:08:31 -0200 From: Marcelo Tosatti To: Radim Krcmar Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Paolo Bonzini , Richard Cochran , Miroslav Lichvar Subject: Re: [patch 3/3] PTP: add kvm PTP driver Message-ID: <20170116170827.GB2501@amt.cnet> References: <20170113120131.086634482@redhat.com> <20170113120319.777765254@redhat.com> <20170113155657.GD22440@potion> <20170113174014.GA9310@amt.cnet> <20170116162653.GA32097@potion> <20170116165411.GA2386@potion> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170116165411.GA2386@potion> User-Agent: Mutt/1.5.21 (2010-09-15) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Mon, 16 Jan 2017 17:09:00 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 16, 2017 at 05:54:11PM +0100, Radim Krcmar wrote: > 2017-01-16 17:26+0100, Radim Krcmar: > > 2017-01-13 15:40-0200, Marcelo Tosatti: > >> On Fri, Jan 13, 2017 at 04:56:58PM +0100, Radim Krcmar wrote: > >> > 2017-01-13 10:01-0200, Marcelo Tosatti: > >>> > + version = pvclock_read_begin(src); > >>> > + > >>> > + ret = kvm_hypercall2(KVM_HC_CLOCK_OFFSET, > >>> > + clock_off_gpa, > >>> > + KVM_CLOCK_OFFSET_WALLCLOCK); > >>> > + if (ret != 0) { > >>> > + pr_err("clock offset hypercall ret %lu\n", ret); > >>> > + spin_unlock(&kvm_ptp_lock); > >>> > + preempt_enable_notrace(); > >>> > + return -EOPNOTSUPP; > >>> > + } > >>> > + > >>> > + tspec.tv_sec = clock_off.sec; > >>> > + tspec.tv_nsec = clock_off.nsec; > >>> > + > >>> > + delta = rdtsc_ordered() - clock_off.tsc; > >>> > + > >>> > + offset = pvclock_scale_delta(delta, src->tsc_to_system_mul, > >>> > + src->tsc_shift); > >>> > + > >>> > + } while (pvclock_read_retry(src, version)); > >>> > + > >>> > + preempt_enable_notrace(); > >>> > + > >>> > + tspec.tv_nsec = tspec.tv_nsec + offset; > >>> > + > >>> > + spin_unlock(&kvm_ptp_lock); > >>> > + > >>> > + if (tspec.tv_nsec >= NSEC_PER_SEC) { > >>> > + u64 secs = tspec.tv_nsec; > >>> > + > >>> > + tspec.tv_nsec = do_div(secs, NSEC_PER_SEC); > >>> > + tspec.tv_sec += secs; > >>> > + } > >>> > + > >>> > + memcpy(ts, &tspec, sizeof(struct timespec64)); > >>> > >>> But the whole idea is of improving the time by reading tsc a bit later > >>> is just weird ... why is it better to provide > >>> > >>> tsc + x, time + tsc_delta_to_time(x) > >>> > >>> than just > >>> > >>> tsc, time > >>> > >>> ? > >> > >> Because you want to calculate the value of the host realtime clock > >> at the moment of ptp_kvm_gettime. > >> > >> We do: > >> > >> 1. kvm_hypercall. > >> 2. get {sec, nsec, guest_tsc}. > >> 3. kvm_hypercall returns. > >> 4. delay = rdtsc() - guest_tsc. > >> > >> Where delay is the delta (measured with the TSC) between points 2 and 4. > > > > I see now ... the PTP interface is just not good for our purposes. > > There is getcrosststamp() callback in PTP, which seems to be exactly > what we want when pairing with TSC, so the pvclock delay fixup can be > dropped when using it. What pvclock delay fixup you refer to? The "rdtsc() - clock_offset.tsc" part? You can't drop it, because if you do then your "host realtime clock read" will be behind by "rdtsc() - clock_offset.tsc" TSC cycles. We want the highest precision as possible.