linux-kernel - RE: [PATCH v2 08/10] clocksource/drivers/hyper-v: Handle sched

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Date:   Tue, 2 Mar 2021 01:38:54 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "hpa@...or.com" <hpa@...or.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: RE: [PATCH v2 08/10] clocksource/drivers/hyper-v: Handle sched_clock
 differences inline

From: Daniel Lezcano <daniel.lezcano@...aro.org> Sent: Monday, March 1, 2021 6:25 AM
> 
> On 01/03/2021 02:15, Michael Kelley wrote:
> > While the Hyper-V Reference TSC code is architecture neutral, the
> > pv_ops.time.sched_clock() function is implemented for x86/x64, but not
> > for ARM64. Current code calls a utility function under arch/x86 (and
> > coming, under arch/arm64) to handle the difference.
> >
> > Change this approach to handle the difference inline based on whether
> > GENERIC_SCHED_CLOCK is present.  The new approach removes code under
> > arch/* since the difference is tied more to the specifics of the Linux
> > implementation than to the architecture.
> >
> > No functional change.
> >
> > Signed-off-by: Michael Kelley <mikelley@...rosoft.com>
> > Reviewed-by: Boqun Feng <boqun.feng@...il.com>
> > ---
> 
> [ ... ]
> 
> > +/*
> > + * Reference to pv_ops must be inline so objtool
> > + * detection of noinstr violations can work correctly.
> > + */
> > +static __always_inline void hv_setup_sched_clock(void *sched_clock)
> > +{
> > +#ifdef CONFIG_GENERIC_SCHED_CLOCK
> > +	/*
> > +	 * We're on an architecture with generic sched clock (not x86/x64).
> > +	 * The Hyper-V sched clock read function returns nanoseconds, not
> > +	 * the normal 100ns units of the Hyper-V synthetic clock.
> > +	 */
> > +	sched_clock_register(sched_clock, 64, NSEC_PER_SEC);
> > +#else
> > +#ifdef CONFIG_PARAVIRT
> > +	/* We're on x86/x64 *and* using PV ops */
> > +	pv_ops.time.sched_clock = sched_clock;
> > +#endif
> > +#endif
> > +}
> Please refer to:
> 
> Documentation/process/coding-style.rst
> 
> Section 21)
> 
> [ ... ]
> 
> Prefer to compile out entire functions, rather than portions of
> functions or portions of expressions.
> 
> [ ... ]
> 

OK.  I'll rework the #ifdef in v3 of the patch set.  Is the following
the preferred approach?

#ifdef CONFIG_GENERIC_SCHED_CLOCK
static __always_inline void hv_setup_sched_clock(void *sched_clock)
{
	sched_clock_register(sched_clock, 64 NSEC_PER_SEC);
}
#else
#ifdef CONFIG_PARAVIRT
static __always_inline void hv_setup_sched_clock(void *sched_clock)
{
	pv_ops.time.sched_clock = sched_clock:
}
#else
static __always_inline void hv_setup_sched_clock(void *sched_clock) {}
#endif
#endif

Michael