Date:   Wed, 26 Jun 2019 12:08:29 +0100
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-arch@...r.kernel.org,
        LAK <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mips@...r.kernel.org,
        linux-kselftest@...r.kernel.org, catalin.marinas@....com,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>, linux@...linux.org.uk,
        Ralf Baechle <ralf@...ux-mips.org>, paul.burton@...s.com,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        salyzyn@...roid.com, pcc@...gle.com, shuah@...nel.org,
        0x7f454c46@...il.com, linux@...musvillemoes.dk,
        huw@...eweavers.com, sthotton@...vell.com, andre.przywara@....com,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: lib/vdso: Make delta calculation work correctly

Hi Thomas,

On 26/06/2019 11:02, Thomas Gleixner wrote:
> The x86 vdso implementation on which the generic vdso library is based on
> has subtle (unfortunately undocumented) twists:
> 
>  1) The code assumes that the clocksource mask is U64_MAX which means that
>     no bits are masked. Which is true for any valid x86 VDSO clocksource.
>     Stupidly it still did the mask operation for no reason and at the wrong
>     place right after reading the clocksource.
> 
>  2) It contains a sanity check to catch the case where slightly
>     unsynchronized TSC values can be overserved which would cause the delta
>     calculation to make a huge jump. It therefore checks whether the
>     current TSC value is larger than the value on which the current
>     conversion is based on. If it's not larger the base value is used to
>     prevent time jumps.
> 
> #1 Is not only stupid for the X86 case because it does the masking for no
> reason it is also completely wrong for clocksources with a smaller mask
> which can legitimately wrap around during a conversion period. The core
> timekeeping code does it correct by applying the mask after the delta
> calculation:
> 
> 	(now - base) & mask
> 
> #2 is equally broken for clocksources which have smaller masks and can wrap
> around during a conversion period because there the now > base check is
> just wrong and causes stale time stamps and time going backwards issues.
> 
> Unbreak it by:
> 
>   1) Removing the mask operation from the clocksource read which makes the
>      fallback detection work for all clocksources
> 
>   2) Replacing the conditional delta calculation with a overrideable inline
>      function.
> 
> #2 could reuse clocksource_delta() from the timekeeping code but that
> results in a significant performance hit for the x86 VSDO. The timekeeping
> core code must have the non optimized version as it has to operate
> correctly with clocksources which have smaller masks as well to handle the
> case where TSC is discarded as timekeeper clocksource and replaced by HPET
> or pmtimer. For the VDSO there is no replacement clocksource. If TSC is
> unusable the syscall is enforced which does the right thing.
> 
> To accomodate to the needs of various architectures provide an overrideable
> inline function which defaults to the regular delta calculation with
> masking:
> 
> 	(now - base) & mask
> 
> Override it for x86 with the non-masking and checking version.
> 
> This unbreaks the ARM64 syscall fallback operation, allows to use
> clocksources with arbitrary width and preserves the performance
> optimization for x86.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

A part a typo that leads to compilation errors on non-x86 platforms the rest
looks fine by me.

I tested it on arm64 and behaves correctly.

With this:

Reviewed-by: Vincenzo Frascino <vincenzo.frascino@....com>

> ---
>  arch/x86/include/asm/vdso/gettimeofday.h |   27 +++++++++++++++++++++++++++
>  lib/vdso/gettimeofday.c                  |   19 +++++++++++++++----
>  2 files changed, 42 insertions(+), 4 deletions(-)
> 
> --- a/arch/x86/include/asm/vdso/gettimeofday.h
> +++ b/arch/x86/include/asm/vdso/gettimeofday.h
> @@ -229,6 +229,33 @@ static __always_inline const struct vdso
>  	return __vdso_data;
>  }
>  
> +/*
> + * x86 specific delta calculation.
> + *
> + * The regular implementation assumes that clocksource reads are globally
> + * monotonic. The TSC can be slightly off across sockets which can cause
> + * the regular delta calculation (@cycles - @last) to return a huge time
> + * jump.
> + *
> + * Therefore it needs to be verified that @cycles are greater than
> + * @last. If not then use @last, which is the base time of the current
> + * conversion period.
> + *
> + * This variant also removes the masking of the subtraction because the
> + * clocksource mask of all VDSO capable clocksources on x86 is U64_MAX
> + * which would result in a pointless operation. The compiler cannot
> + * optimize it away as the mask comes from the vdso data and is not compile
> + * time constant.
> + */
> +static __always_inline
> +u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult)
> +{
> +	if (cycles > last)
> +		return (cycles - last) * mult;
> +	return 0;
> +}
> +#define vdso_calc_delta vdso_calc_delta
> +
>  #endif /* !__ASSEMBLY__ */
>  
>  #endif /* __ASM_VDSO_GETTIMEOFDAY_H */
> --- a/lib/vdso/gettimeofday.c
> +++ b/lib/vdso/gettimeofday.c
> @@ -26,6 +26,18 @@
>  #include <asm/vdso/gettimeofday.h>
>  #endif /* ENABLE_COMPAT_VDSO */
>  
> +#ifndef vdso_calc_delta
> +/*
> + * Default implementation which works for all sane clocksources. That
> + * obviously excludes x86/TSC.
> + */
> +static __always_inline
> +u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult)
> +{
> +	return ((cyles - last) & mask) * mult;

Typo here:

s/cyles/cycles/

> +}
> +#endif
> +
>  static int do_hres(const struct vdso_data *vd, clockid_t clk,
>  		   struct __kernel_timespec *ts)
>  {
> @@ -35,14 +47,13 @@ static int do_hres(const struct vdso_dat
>  
>  	do {
>  		seq = vdso_read_begin(vd);
> -		cycles = __arch_get_hw_counter(vd->clock_mode) &
> -			vd->mask;
> +		cycles = __arch_get_hw_counter(vd->clock_mode);
>  		ns = vdso_ts->nsec;
>  		last = vd->cycle_last;
>  		if (unlikely((s64)cycles < 0))
>  			return clock_gettime_fallback(clk, ts);
> -		if (cycles > last)
> -			ns += (cycles - last) * vd->mult;
> +
> +		ns += vdso_calc_delta(cycles, last, vd->mask, vd->mult);
>  		ns >>= vd->shift;
>  		sec = vdso_ts->sec;
>  	} while (unlikely(vdso_read_retry(vd, seq)));
> 

-- 
Regards,
Vincenzo

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