linux-kernel - Re: [PATCH v13 07/18] x86/xen/time: output xen sched

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Date:   Tue, 17 Jul 2018 21:59:15 -0400
From:   Pavel Tatashin <pasha.tatashin@...cle.com>
To:     boris.ostrovsky@...cle.com
Cc:     Steven Sistare <steven.sistare@...cle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        linux@...linux.org.uk, schwidefsky@...ibm.com,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        John Stultz <john.stultz@...aro.org>, sboyd@...eaurora.org,
        x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
        douly.fnst@...fujitsu.com, peterz@...radead.org, prarit@...hat.com,
        feng.tang@...el.com, Petr Mladek <pmladek@...e.com>,
        gnomes@...rguk.ukuu.org.uk, linux-s390@...r.kernel.org,
        jgross@...e.com
Subject: Re: [PATCH v13 07/18] x86/xen/time: output xen sched_clock time from 0

> > +static u64 xen_sched_clock(void)
> > +{
> > +     return xen_clocksource_read() - xen_sched_clock_offset;
> > +}
>
>
> Should other invocations of xen_clocksource_read() also be offset?
>

I do not believe so. Look in arch/x86/kernel/kvmclock.c
kvm_clock_get_cycles() is not normalized for instance. We need to
normalize kvm_sched_clock_read() and xen_sched_clock(), because it is
used by printk() as a sched_clock() prefix. So not to be confusing we
print time starting from 0, but the other instances where
xen_clocksource_read()  is used do not need that.

Thank you,
Pavel