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

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Date:   Fri, 20 Jan 2017 19:08:49 +0100
From:   Radim Krcmar <rkrcmar@...hat.com>
To:     Marcelo Tosatti <mtosatti@...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

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?