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

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Date:   Wed, 18 Jan 2017 16:45:07 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Miroslav Lichvar <mlichvar@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Richard Cochran <richardcochran@...il.com>
Subject: Re: [patch 3/3] PTP: add kvm PTP driver



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