linux-kernel - Re: [PATCH RFC V1 0/5] Rationalize time keeping

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Date:	Thu, 03 May 2012 08:48:44 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Richard Cochran <richardcochran@...il.com>
CC:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RFC V1 0/5] Rationalize time keeping

On 05/03/2012 12:02 AM, Richard Cochran wrote:
> On Mon, Apr 30, 2012 at 01:56:16PM -0700, John Stultz wrote:
>> No. So, on architectures that support vsyscalls/vdso (x86_64,
>> powerpc, ia64, and maybe a few others) getnstimeofday() is really
>> only an internal interface for in-kernel access. Userland uses the
>> vsyscall/vdso interface to be able to read the time completely from
>> userland context (with no syscall overhead). Since this is done in
>> different ways for each architecture, you need to export the proper
>> information out via update_vsyscall() and also update the
>> arch-specific vsyscall gettimeofday paths (which is non-trivial, as
>> some arches are implemented in asm, etc - my sympathies here, its a
>> pain).
> Okay, so now I understand the vDSO page thingy. Help me please to
> understand exactly which architectures would need changes for my
> proposal.
>
> The only archs exporting time variables/functions through vDSO are
> those which define CONFIG_GENERIC_TIME_VSYSCALL=y, namely:
>
> - ia64
> - powerpc 64 and 32 bit
> - s390    64 and 32 bit
> - x86     64 bit only **
>
>    ** But 32 guest running in a 64 host also has time in the vDSO?!?
   Yes, but at least on x86 its just one implementation that needs to be 
modified.

>
> Did I get that right?
That looks right to me.

thanks
-john

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