Re: [rfc 08/45] cpu alloc: x86 support

Christoph Lameter
2007-11-21T02:08:50Z

On Wed, 21 Nov 2007, Andi Kleen wrote:

> On Wednesday 21 November 2007 02:16:11 Christoph Lameter wrote:
> > But one can subtract too...
>
> The linker cannot subtract (unless you add a new relocation types)

The compiler knows and emits assembly to compensate.

> All you need is a 2MB area (16MB is too large if you really
> want 16k CPUs someday) somewhere in the -2GB or probably better
> in +2GB. Then the linker puts stuff in there and you use
> the offsets for referencing relative to %gs.

2MB * 16k = 32GB. Even with 4k cpus we will have 2M * 4k = 8GB both do
not fit in the 2GB area.

The offset relative to %gs cannot be used if you have a loop and are
calculating the addresses for all instances. That is what we are talking
about. The CPU_xxx operations that are using the %gs register are fine and
are not affected by the changes we are discussing.

> Then for all CPUs (including CPU #0) you put the real mapping
> somewhere else, copy the reference data there (which also doesn't need
> to be on the offset the linker assigned, just on a constant offset
> from it somewhere in the normal kernel data) and off you go.

Real mapping? We have constant offsets after this patchset. I do not get
what you are planning here.

> Then the reference data would be initdata and eventually freed.
> That is similar to how the current per cpu data works.

Yes that is also how the current patchset works. I just do not understand
what you want changed.