linux-kernel - Re: [PATCH] Add VDSO time function support for x86 32-bit kernel

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Message-ID: <1d3061cb-76d0-4e42-9b75-a975b05384ec@email.android.com>
Date:	Wed, 12 Dec 2012 22:14:25 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Stefani Seibold <stefani@...bold.net>
CC:	linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
	mingo@...hat.com, ak@...ux.intel.com, aarcange@...hat.com,
	john.stultz@...aro.org, luto@...capital.net
Subject: Re: [PATCH] Add VDSO time function support for x86 32-bit kernel

This is too late for 3.8 anyway, so there is time to make it work correctly before tge 3.9 merge window anyway.  After this merge window is over I may pull tjis into a testing branch, but compat support is a precondition.

The vdso is only optional if you build in backwards compatibility anyway, and software has a right to expect a specific numeric kernel version to export a single ABI.

Stefani Seibold <stefani@...bold.net> wrote:

>Am Mittwoch, den 12.12.2012, 15:34 -0800 schrieb H. Peter Anvin:
>> On 12/12/2012 12:19 PM, stefani@...bold.net wrote:
>> > diff --git a/arch/x86/vdso/vdso32/vclock_gettime.c
>b/arch/x86/vdso/vdso32/vclock_gettime.c
>> > new file mode 100644
>> > index 0000000..c9a1909
>> > --- /dev/null
>> > +++ b/arch/x86/vdso/vdso32/vclock_gettime.c
>> > @@ -0,0 +1,7 @@
>> > +/*
>> > + * since vgtod layout differs between X86_64 and x86_32, it is not
>possible to
>> > + * provide a 32 bit vclock with a 64 bit kernel
>> > + */
>> > +#ifdef CONFIG_X86_32
>> > +#include "../vclock_gettime.c"
>> > +#endif
>> 
>> This is where this goes fail.  Sorry, it is not acceptable to
>introduce 
>> an ABI on x86-32 without providing it also on x86-64 in compatibility
>mode.
>> 
>> I also don't believe it is not possible... it might require some more
>
>> cleverness; perhaps we need to do the 32-bit vgtod in such a way that
>it 
>> *is* compatible with 64 bits.
>> 
>
>The comment is ambiguous:
>
>Since vgtod layout differs between X86_64 and x86_32 AND the vgtod is
>not inside the accessible address space of a 32 bit program, it is
>CURRENTLY not possible to provide a 32 bit vclock with a 64 bit kernel
>
>As i understand VDSO it is an alternativ way, so if there is no support
>for it, there must be a fall back to the system call interface in the
>program or lib, which tries to use it.
>
>So there is no drawback for 32 bit programs running on a 64 bit kernel.
>
>
>I think this feature is not so important and can implemented in a
>subsequent patch, because a 64 bit kernel system mostly runs 64 bit
>programs. Let us fix this things step by step.
>
>- Stefani

-- 
Sent from my mobile phone. Please excuse brevity and lack of formatting.
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