RE: [PATCH v2 07/10] clocksource/drivers/hyper-v: Handle vDSO differences inline

From: Michael Kelley <mikelley@microsoft.com>
To: Daniel Lezcano <daniel.lezcano@linaro.org>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	KY Srinivasan <kys@microsoft.com>,
	"wei.liu@kernel.org" <wei.liu@kernel.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"bp@alien8.de" <bp@alien8.de>, "hpa@zytor.com" <hpa@zytor.com>,
	"arnd@arndb.de" <arnd@arndb.de>,
	"linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>
Subject: RE: [PATCH v2 07/10] clocksource/drivers/hyper-v: Handle vDSO differences inline
Date: Tue, 2 Mar 2021 01:29:10 +0000	[thread overview]
Message-ID: <MWHPR21MB15930DD833E49415610C021DD7999@MWHPR21MB1593.namprd21.prod.outlook.com> (raw)
In-Reply-To: <42dc252a-b09a-afeb-6792-9b77669c16e9@linaro.org>

From: Daniel Lezcano <daniel.lezcano@linaro.org> Sent: Monday, March 1, 2021 4:22 AM
> 
> On 01/03/2021 02:15, Michael Kelley wrote:
> > While the driver for the Hyper-V Reference TSC and STIMERs is architecture
> > neutral, vDSO is implemented for x86/x64, but not for ARM64.  Current code
> > calls into utility functions under arch/x86 (and coming, under arch/arm64)
> > to handle the difference.
> >
> > Change this approach to handle the difference inline based on whether
> > VDSO_CLOCK_MODE_HVCLOCK 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.
> 
> A suggestion below
> 
> 
> > Signed-off-by: Michael Kelley <mikelley@microsoft.com>
> > Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> > ---
> >  arch/x86/include/asm/mshyperv.h    |  4 ----
> >  drivers/clocksource/hyperv_timer.c | 10 ++++++++--
> >  2 files changed, 8 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
> > index c73c127..5e5e08aa 100644
> > --- a/drivers/clocksource/hyperv_timer.c
> > +++ b/drivers/clocksource/hyperv_timer.c
> > @@ -372,7 +372,9 @@ static void resume_hv_clock_tsc(struct clocksource *arg)
> >
> >  static int hv_cs_enable(struct clocksource *cs)
> 
> static __maybe_unused int hv_cs_enable(struct clocksource *cs)
> 
> >  {
> > -	hv_enable_vdso_clocksource();
> > +#ifdef VDSO_CLOCKMODE_HVCLOCK
> > +	vclocks_set_used(VDSO_CLOCKMODE_HVCLOCK);
> > +#endif
> >  	return 0;
> >  }
> >
> > @@ -385,6 +387,11 @@ static int hv_cs_enable(struct clocksource *cs)
> >  	.suspend= suspend_hv_clock_tsc,
> >  	.resume	= resume_hv_clock_tsc,
> >  	.enable = hv_cs_enable,
> > +#ifdef VDSO_CLOCKMODE_HVCLOCK
> > +	.vdso_clock_mode = VDSO_CLOCKMODE_HVCLOCK,
> > +#else
> > +	.vdso_clock_mode = VDSO_CLOCKMODE_NONE,
> > +#endif
> 
> #ifdef VDSO_CLOCKMODE_HVCLOCK
> 	.enable = hv_cs_enable,
> 	.vdso_clock_mode = VDSO_CLOCKMODE_HVCLOCK,
> #else
> 	.vdso_clock_mode = VDSO_CLOCKMODE_NONE,
> #endif
> 

Is there any particular benefit (that I might not be recognizing)
to having the .enable function be NULL vs. a function that
does nothing?  I can see the handful of places where the
.enable function is invoked, and there doesn't seem to be
much difference.

In any case, I have no problem with making the change in
a v3 of the patch set.

Michael