Re: [patch 3/3] PTP: add kvm PTP driver

Paolo Bonzini
2017-01-18T15:52:35Z

On 18/01/2017 15:50, Marcelo Tosatti wrote:
>> Interesting idea! For this to work, KVM needs to implement
>> getcrosstimestamp and ptp_chardev.c can then add an alternative
>> implementation of PTP_SYS_OFFSET, based on precise cross timestamps.
>>
>> Something like
>>
>> for (i = 0; i <= sysoff->n_samples; i++) {
>> // ... call getcrosststamp ...
>> sysns = ktime_to_ns(xtstamp.sys_realtime);
>> if (i > 0) {
>> devns = ktime_to_ns(xtstamp.device);
>> devns -= (sysns - prev_sysns) / 2;
>> devts = ns_to_timespec(devns);
>> pct->sec = devts.tv_sec;
>> pct->nsec = devts.tv_nsec;
>> pct++;
>> }
>> systs = ns_to_timespec(sysns);
>> pct->sec = ts.tv_sec;
>> pct->nsec = ts.tv_nsec;
>> pct++;
>> prev_sysns = sysns;
>> }
>>
>> Marcelo, can you give it a try?
>
> Can convert fine, but problem is the simultaneous read
> of host and guest clocks.

Could the TSC from the hypercall be applied to kvmclock to do this? My
understanding is that get_device_system_crosststamp (which is used in
the sole in-tree implementation of getcrosststamp) already contains all
the logic to do that.

Paolo