linux-kernel - Re: [RFC 6/6] arm64: hyperv: Enable vDSO

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Message-ID: <20191218054732.GK97412@debian-boqun.qqnc3lrjykvubdpftowmye0fmh.lx.internal.cloudapp.net>
Date:   Wed, 18 Dec 2019 13:47:32 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Michael Kelley <mikelley@...rosoft.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        xen-devel@...ts.xenproject.org,
        Stefano Stabellini <sstabellini@...nel.org>,
        linux-hyperv@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 6/6] arm64: hyperv: Enable vDSO

On Tue, Dec 17, 2019 at 03:10:16PM +0100, Vitaly Kuznetsov wrote:
> Boqun Feng <boqun.feng@...il.com> writes:
> 
> > Similar to x86, add a new vclock_mode VCLOCK_HVCLOCK, and reuse the
> > hv_read_tsc_page() for userspace to read tsc page clocksource.
> >
> > Signed-off-by: Boqun Feng (Microsoft) <boqun.feng@...il.com>
> > ---
> >  arch/arm64/include/asm/clocksource.h       |  3 ++-
> >  arch/arm64/include/asm/mshyperv.h          |  2 +-
> >  arch/arm64/include/asm/vdso/gettimeofday.h | 19 +++++++++++++++++++
> >  3 files changed, 22 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/clocksource.h b/arch/arm64/include/asm/clocksource.h
> > index fbe80057468c..c6acd45fe748 100644
> > --- a/arch/arm64/include/asm/clocksource.h
> > +++ b/arch/arm64/include/asm/clocksource.h
> > @@ -4,7 +4,8 @@
> >  
> >  #define VCLOCK_NONE	0	/* No vDSO clock available.		*/
> >  #define VCLOCK_CNTVCT	1	/* vDSO should use cntvcnt		*/
> > -#define VCLOCK_MAX	1
> > +#define VCLOCK_HVCLOCK	2	/* vDSO should use vread_hvclock()	*/
> > +#define VCLOCK_MAX	2
> >  
> >  struct arch_clocksource_data {
> >  	int vclock_mode;
> > diff --git a/arch/arm64/include/asm/mshyperv.h b/arch/arm64/include/asm/mshyperv.h
> > index 0afb00e3501d..7c85dd816dca 100644
> > --- a/arch/arm64/include/asm/mshyperv.h
> > +++ b/arch/arm64/include/asm/mshyperv.h
> > @@ -90,7 +90,7 @@ extern void hv_get_vpreg_128(u32 reg, struct hv_get_vp_register_output *result);
> >  #define hv_set_reference_tsc(val) \
> >  		hv_set_vpreg(HV_REGISTER_REFERENCE_TSC, val)
> >  #define hv_set_clocksource_vdso(val) \
> > -		((val).archdata.vclock_mode = VCLOCK_NONE)
> > +		((val).archdata.vclock_mode = VCLOCK_HVCLOCK)
> >  
> >  #if IS_ENABLED(CONFIG_HYPERV)
> >  #define hv_enable_stimer0_percpu_irq(irq)	enable_percpu_irq(irq, 0)
> > diff --git a/arch/arm64/include/asm/vdso/gettimeofday.h b/arch/arm64/include/asm/vdso/gettimeofday.h
> > index e6e3fe0488c7..7e689b903f4d 100644
> > --- a/arch/arm64/include/asm/vdso/gettimeofday.h
> > +++ b/arch/arm64/include/asm/vdso/gettimeofday.h
> > @@ -67,6 +67,20 @@ int clock_getres_fallback(clockid_t _clkid, struct __kernel_timespec *_ts)
> >  	return ret;
> >  }
> >  
> > +#ifdef CONFIG_HYPERV_TIMER
> > +/* This will override the default hv_get_raw_timer() */
> > +#define hv_get_raw_timer() __arch_counter_get_cntvct()
> > +#include <clocksource/hyperv_timer.h>
> > +
> > +extern struct ms_hyperv_tsc_page
> > +_hvclock_page __attribute__((visibility("hidden")));
> > +
> > +static u64 vread_hvclock(void)
> > +{
> > +	return hv_read_tsc_page(&_hvclock_page);
> > +}
> > +#endif
> 
> The function is almost the same on x86 (&_hvclock_page ->
> &hvclock_page), would it maybe make sense to move this to arch neutral
> clocksource/hyperv_timer.h?
> 

I'm not sure whether the underscore matters in the vDSO data symbol, so
I follow the architectural name convention. If the leading underscore
doesn't have special purpose I'm happy to move this to arch neutral
header file.

> > +
> >  static __always_inline u64 __arch_get_hw_counter(s32 clock_mode)
> >  {
> >  	u64 res;
> > @@ -78,6 +92,11 @@ static __always_inline u64 __arch_get_hw_counter(s32 clock_mode)
> >  	if (clock_mode == VCLOCK_NONE)
> >  		return __VDSO_USE_SYSCALL;
> >  
> > +#ifdef CONFIG_HYPERV_TIMER
> > +	if (likely(clock_mode == VCLOCK_HVCLOCK))
> > +		return vread_hvclock();
> 
> I'm not sure likely() is justified here: it'll make ALL builds which
> enable CONFIG_HYPERV_TIMER (e.g. distro kernels) to prefer
> VCLOCK_HVCLOCK, even if the kernel is not running on Hyper-V.
> 

Make sense. Thanks for pointing this out! I will change it in the next
version.

Regards,
Boqun

> > +#endif
> > +
> >  	/*
> >  	 * This isb() is required to prevent that the counter value
> >  	 * is speculated.
> 
> -- 
> Vitaly
>