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

Jan Beulich
2014-11-05T17:04:51Z

>>> "H. Peter Anvin" <hpa@zytor.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