Re: [patch 5/5] PTP: add kvm PTP driver

[Paolo Bonzini]

On 20/01/2017 19:08, 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?

get_device_system_crosststamp handles it, that's why there is a
clocksource field in system_counter.

Paolo

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