linux-kernel - Re: [PATCH] x86: relocs is only used with 32bit

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Message-ID: <m1mybpt5t8.fsf@fess.ebiederm.org>
Date:	Fri, 13 Mar 2009 15:29:39 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: relocs is only used with 32bit

"H. Peter Anvin" <hpa@...or.com> writes:

> Hi Yinghai,
>
> In general I prefer minimizing the conditional sections rather than
> maximizing them.  If nothing else, it catches errors earlier that way,
> and makes it easier to eventually eliminate them entirely.

Makes sense.  Although in this case that rule appears like it
can't even execute on x86_64.

The architectural differences result in different tradeoffs so the
relocation design is completely different on x86_64 and x86_32.

On x86_64 we just change the underlying page tables to point
at different physical addresses and have a fixed virtual address.
Since x86_64 is best with short (< 2GB) offsets that is a fairly
optimal.

On x86_32 where address space is precious and it doesn't really
matter where the kernel lives we take a 5% or so size penalty
to hold relocations in the binary and update the kernel at boot
time so everything assumes we run at the loaded address.

With the result that we need a link time helper on x86_32 and
not on x86_64.

Eric

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