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

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Date:	Thu, 3 May 2012 09:02:07 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RFC V1 0/5] Rationalize time keeping

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?!?

Did I get that right?

Thanks,
Richard


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