linux-kernel - Re: [patch 3/3] PTP: add kvm PTP driver

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Message-ID: <20170116165411.GA2386@potion>
Date:   Mon, 16 Jan 2017 17:54:11 +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 3/3] PTP: add kvm PTP driver

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.