lkml.org 
[lkml]   [2008]   [Jul]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC 00/15] x86_64: Optimize percpu accesses
Eric W. Biederman wrote:
>>>
>> CONFIG_PHYSICAL_START rather. And no, it can't be zero! Realistically we
>> should make it 16 MB by default (currently 2 MB), to keep the DMA zone clear.
>
> Also on x86_64 CONFIG_PHYSICAL_START is irrelevant as the kernel text segment
> is liked at a fixed address -2G and the option only determines the virtual
> to physical address mapping.
>

No, it's not irrelevant; we currently base the kernel at virtual address
-2 GB (KERNEL_IMAGE_START) + CONFIG_PHYSICAL_START, in order to have the
proper alignment for large pages.

Now, it probably wouldn't hurt moving KERNEL_IMAGE_START up a bit to
have low positive values safer to use.

> That said the idea may not be too far off.
>
> Potentially we could put the percpu area at our fixed -2G address and then
> we have a constant (instead of an address) we could subtract from this address.

We can't put it at -2 GB since the offset +40 for the stack sentinel is
hard-coded into gcc. This leaves growing upward from +48 (or another
small positive number), or growing down from zero (or +40) as realistic
options.

Unfortunately, GNU ld handles grow-down not at all.

-hpa


\
 
 \ /
  Last update: 2008-07-10 00:33    [W:0.656 / U:0.164 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site