From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: [patch 4/5] PTP: add PTP_SYS_OFFSET emulation via cross
 timestamps infrastructure
Date: Fri, 20 Jan 2017 11:52:58 -0200
Message-ID: <20170120135254.GA6350@amt.cnet>
References: <20170120122025.665985919@redhat.com>
 <20170120122503.746158230@redhat.com>
 <48bb2650-ed00-ec07-31bf-8780d3ab5568@redhat.com>
 <20170120130711.GA27440@amt.cnet>
 <2d213ad9-fa40-1f1e-90a9-404764969d35@redhat.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Radim Krcmar <rkrcmar@redhat.com>,
        Richard Cochran <richardcochran@gmail.com>,
        Miroslav Lichvar <mlichvar@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
Content-Disposition: inline
In-Reply-To: <2d213ad9-fa40-1f1e-90a9-404764969d35@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

On Fri, Jan 20, 2017 at 02:36:40PM +0100, Paolo Bonzini wrote:
> 
> 
> On 20/01/2017 14:07, Marcelo Tosatti wrote:
> > On Fri, Jan 20, 2017 at 01:55:27PM +0100, Paolo Bonzini wrote:
> >>
> >>
> >> On 20/01/2017 13:20, Marcelo Tosatti wrote:
> >>>  kernel/time/timekeeping.c        |   79 +++++++++++++++++++++++++++++++++++++++
> >>
> >> Why not leave this in drivers/ptp/ptp_chardev.c?
> > 
> > timekeeper_lock
> 
> Why does emulate_ptp_sys_offset need it, if the current PTP_SYS_OFFSET
> code doesn't? 

Because if time is adjusted while you are taking the samples, 
the mean can return non existant values:

1) take sample1  (realtime = 2000)
2) userspace changes realtime (realtime = 100)
3) 2100/2 = 1050

However that 1050 value never existed, before or after
userspace changed realtime.
Such behaviour does not exist with PTP_SYS_OFFSET, because 
taking getnstimeofday64 is serialized against time changes.

I am not sure whether returning such bizzare values is fine, to 
drop the lock.

Hum... i think it must be because userspace will consider
the new values after realtime is changed as correct.

>  Is the latency acceptable (considering this is a raw spin
> lock) or is there a seqlock that we can use instead (such as tk_core.seq
> like in get_device_system_crosststamp)?

Well can move it after the ->getcrosststamp loop. 
I'll just drop the spinlock and document the behaviour.