linux-kernel - Re: [tip:x86/boot] x86-64: Use RIP-relative addressing for most per-CPU accesses

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Message-ID: <545932C3.3080907@zytor.com>
Date:	Tue, 04 Nov 2014 12:10:43 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	mingo@...nel.org, linux-kernel@...r.kernel.org, JBeulich@...e.com,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/boot] x86-64: Use RIP-relative addressing for most per-CPU
 accesses

On 11/04/2014 11:45 AM, tip-bot for Jan Beulich wrote:
> Commit-ID:  97b67ae559947f1e208439a1bf6a734da3087006
> Gitweb:     http://git.kernel.org/tip/97b67ae559947f1e208439a1bf6a734da3087006
> Author:     Jan Beulich <JBeulich@...e.com>
> AuthorDate: Tue, 4 Nov 2014 08:50:48 +0000
> Committer:  Thomas Gleixner <tglx@...utronix.de>
> CommitDate: Tue, 4 Nov 2014 20:43:14 +0100
> 
> x86-64: Use RIP-relative addressing for most per-CPU accesses
> 
> Observing that per-CPU data (in the SMP case) is reachable by
> exploiting 64-bit address wraparound (building on the default kernel
> load address being at 16Mb), the one byte shorter RIP-relative
> addressing form can be used for most per-CPU accesses. The one
> exception are the "stable" reads, where the use of the "P" operand
> modifier prevents the compiler from using RIP-relative addressing, but
> is unavoidable due to the use of the "p" constraint (side note: with
> gcc 4.9.x the intended effect of this isn't being achieved anymore,
> see gcc bug 63637).
> 
> With the dependency on the minimum kernel load address, arbitrarily
> low values for CONFIG_PHYSICAL_START are now no longer possible. A
> link time assertion is being added, directing to the need to increase
> that value when it triggers.
> 

This description makes very little sense.  What matters here is the
virtual address, which is always >= -2 GB.

	-hpa


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