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Date: Mon, 7 Jul 2008 10:55:10 -0500
From: Nathan Lynch <ntl@pobox.com>
To: Mikael Pettersson <mikpe@it.uu.se>
Subject: Re: [PATCH 1/2] elf loader support for auxvec base platform string
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Cc: Benjamin, linux-kernel@vger.kernel.org, linuxppc-dev@ozlabs.org,
	Paul Mackerras <paulus@samba.org>, Roland McGrath <roland@redhat.com>
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Mikael Pettersson wrote:
> Nathan Lynch writes:
>  > Some IBM POWER-based platforms have the ability to run in a
>  > mode which mostly appears to the OS as a different processor from the
>  > actual hardware.  For example, a Power6 system may appear to be a
>  > Power5+, which makes the AT_PLATFORM value "power5+".
>  > 
>  > However, some applications (virtual machines, optimized libraries) can
>  > benefit from knowledge of the underlying CPU model.  A new aux vector
>  > entry, AT_BASE_PLATFORM, will denote the actual hardware.  For
>  > example, on a Power6 system in Power5+ compatibility mode, AT_PLATFORM
>  > will be "power5+" and AT_BASE_PLATFORM will be "power6".
> 
> Why on earth would you ever want AT_PLATFORM to differ from AT_BASE_PLATFORM?
> In cases that matter you admit that AT_BASE_PLATFORM takes precedence,
> so why involve a fake lame not-quite-the-platform in the first place?
> 
> Workaround for buggy software?

My apologies, I did not explain the motivation well.

The idea is that while AT_PLATFORM indicates the instruction set
supported, AT_BASE_PLATFORM indicates the underlying
microarchitecture.  It's not a matter of buggy software, or of one
value taking precedence over the other.