linux-kernel - Re: [PATCH 0/6][RFC] Rework vsyscall to avoid truncation/rounding issue in timekeeping core

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Date:	Tue, 18 Sep 2012 11:17:23 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	John Stultz <john.stultz@...aro.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	Paul Mackerras <paulus@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Paul Turner <pjt@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/6][RFC] Rework vsyscall to avoid truncation/rounding
 issue in timekeeping core

On Tue, Sep 18, 2012 at 11:02 AM, Richard Cochran
<richardcochran@...il.com> wrote:
> On Mon, Sep 17, 2012 at 05:20:41PM -0700, John Stultz wrote:
>> On 09/17/2012 04:49 PM, Andy Lutomirski wrote:
>> >2. There's nothing vsyscall-specific about the code in
>> >vclock_gettime.c.  In fact, the VVAR macro should work just fine in
>> >kernel code.  If you moved all this code into a header, then in-kernel
>> >uses could use it, and maybe even other arches could use it.  Last
>> >time I checked, it seemed like vclock_gettime was considerably faster
>> >than whatever the in-kernel equivalent did.
>> I like the idea of unifying the implementations, but I'd want to
>> know more about why vclock_gettime was faster then the in-kernel
>> getnstimeofday(), since it might be due to the more limited locking
>> (we only update vsyscall data under the vsyscall lock, where as the
>> timekeeper lock is held for the entire execution of
>> update_wall_time()), or some of the optimizations in the vsyscall
>> code is focused on providing timespecs to userland, where as
>> in-kernel we also have to provide ktime_ts.
>
> This there a valid technical reason why each arch has its own vdso
> implementation?

I don't know too much about other arch vdsos.  i386's doesn't have
clock functions.  x32 works exactly like x86-64, except that it
probably involves a bit of addressing mode weirdness.  ia64's is very
strange indeed, I think.

In any case, the VVAR macro is an x86-64-ism, although if it were to
be the beginning of a generic mechanism, #define VVAR(x) (x) would be
a perfectly fine start, I think.

>
> If not, I would suggest that the first step would be to refactor these
> into one C-language header. If this can be shared with kernel code,
> then all the better.

That should be most straightforward to do.

One issue: you can't call a function pointer from vdso code (because
the vdso is in a different place in different processes).  The
vclock_mode stuff would need to be extended to work across
architectures, and the fallback to a real syscall would need to turn
into something else.

--Andy
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