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

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Date:	Wed, 05 Nov 2014 17:04:02 +0000
From:	"Jan Beulich" <jbeulich@...e.com>
To:	<mingo@...nel.org>, <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>,
	<linux-tip-commits@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [tip:x86/boot] x86-64: Use RIP-relative addressing for
 most per-CPU accesses

>>> "H. Peter Anvin" <hpa@...or.com> 11/04/14 9:11 PM >>>
>On 11/04/2014 11:45 AM, tip-bot for Jan Beulich wrote:
>> 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.

But code at or very close to -2Gb won't be able to reach addresses far
enough from address zero; it indeed matters that the lowest code address
is far enough away from -2Gb.

Jan

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