linux-kernel - Re: [PATCH 00/25] Change time_t and clock

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Date:	Wed, 14 May 2014 16:46:29 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Arnd Bergmann <arnd@...db.de>
cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Christoph Hellwig <hch@...radead.org>,
	Ley Foon Tan <lftan@...era.com>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	LeyFoon Tan <lftan.linux@...il.com>,
	Chung-Lin Tang <cltang@...esourcery.com>
Subject: Re: [PATCH 00/25] Change time_t and clock_t to 64 bit

On Wed, 14 May 2014, Arnd Bergmann wrote:

> On Wednesday 14 May 2014 14:21:48 Thomas Gleixner wrote:
> > 
> > So in the 32-on-64 case we'll have two compat variants:
> > 
> > SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
> >                 struct timespec __user *, utime, u32 __user *, uaddr2,
> >                 u32, val3)
> > 
> > COMPAT_SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
> >                 struct compat_timespec __user *, utime, u32 __user *, uaddr2,
> >                 u32, val3)
> > 
> > COMPAT_SYSCALL_DEFINE6(futex64, u32 __user *, uaddr, int, op, u32, val,
> >                 struct timespec64 __user *, utime, u32 __user *, uaddr2,
> >                 u32, val3)
> > 
> > The native 64bit futex64 syscall is mapped to futex.
> 
> I was actually hoping that we could map the compat futex64 to futex
> as well here, since 64-bit timespec and compat timespec64 would be
> the same structure.

Right, that might work with this one, but not for anything which has a
pointer to a timespec and some other argument based on long.

> > I'm curious, whether quite some code, like high frequency timestamps
> > wouldn't be better of with a strict 64 bit nanosecond granular time
> > represenation.
> 
> At least in the kernel, I think ktime_t is already the right type
> to use on both 64-bit and 32-bit architectures as it can be slow to
> extract the seconds portion of 64-bit nanoseconds on a 32-bit machine.

On some of them yes. On i386 the u64 nsec ktime_t variant is way more
efficient.

> FWIW, 64-bit ns gives us 584 years worth of nanoseconds, which
> means none of us or the people we know will be around before this
> becomes a problem ;-)

Indeed.
 
> For the user interface, we can decide which representation to use
> for each syscall individually depending on the needs. We should just
> not have to many different variants. I was going for timespec64
> just because that would allow us to keep the 64-bit kernel ABI
> unchanged.

Right. It's the way of least resistance.

If there are desires from user space to have a new format instead of
blindly timespec64 for certain syscalls, we should really think
discuss that on a case by case basis.

Thanks,

	tglx

 
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