linux-kernel - Re: [patch 5/5] PTP: add kvm PTP driver

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Message-ID: <20170120191030.GA1942@amt.cnet>
Date:   Fri, 20 Jan 2017 17:10:33 -0200
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Radim Krcmar <rkrcmar@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Richard Cochran <richardcochran@...il.com>,
        Miroslav Lichvar <mlichvar@...hat.com>
Subject: Re: [patch 5/5] PTP: add kvm PTP driver

On Fri, Jan 20, 2017 at 07:08:49PM +0100, Radim Krcmar wrote:
> 2017-01-20 13:00-0200, Marcelo Tosatti:
> > On Fri, Jan 20, 2017 at 03:12:56PM +0100, Radim Krcmar wrote:
> >> 2017-01-20 10:20-0200, Marcelo Tosatti:
> >> > +	do {
> >> > +		/*
> >> > +		 * We are measuring the delay between
> >> > +		 * kvm_hypercall and rdtsc using TSC,
> >> > +		 * and converting that delta to
> >> > +		 * tsc_to_system_mul and tsc_shift
> >> > +		 * So any changes to tsc_to_system_mul
> >> > +		 * and tsc_shift in this region
> >> > +		 * invalidate the measurement.
> >> > +		 */
> >> > +		version = pvclock_read_begin(src);
> >> > +
> >> > +		ret = kvm_hypercall2(KVM_HC_CLOCK_PAIRING,
> >> > +				     clock_off_gpa,
> >> > +				     KVM_CLOCK_PAIRING_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;
> >> > +		ret = __pvclock_read_cycles(src, clock_off.tsc);
> >> > +		flags = src->flags;
> >> > +	} while (pvclock_read_retry(src, version));
> >> > +
> >> > +	preempt_enable_notrace();
> >> > +
> >> > +	system_counter->cycles = ret;
> >> > +	system_counter->cs = get_kvmclock_cs();
> >> 
> >> Can't we use clocksource_tsc and just pass the tsc without kvmclock in
> >> the middle?
> > 
> > No, it has to be the kvmclock value.
> 
> What happens if the guest switches from kvmclock to tsc?

The ioctl will return -ENODEV.

>From get_device_system_crosststamp function:

                /*
                 * Verify that the clocksource associated with the
                 * captured
                 * system counter value is the same as the currently
                 * installed
                 * timekeeper clocksource
                 */
                if (tk->tkr_mono.clock != system_counterval.cs)
                        return -ENODEV;