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

[H. Peter Anvin]

Christoph Lameter wrote:
> On Tue, 20 Nov 2007, Andi Kleen wrote:
>
> > > This limitation shouldn't apply to the percpu area, since gs_base can be 
> > > pointed anywhere in the address space -- in effect we're always indirect.
> > The initial reference copy of the percpu area has to be addressed by
> > the linker.
>
> Right that is important for the percpu references that can be folded by 
> the linker in order to avoid address calculations.
>
> > Hmm, in theory since it is not actually used by itself I suppose you could 
> > move it into positive space.
>
> But the positive space is reserved for a processes memory.

But you wouldn't actually *use* this address space.  It's just for the 
linker to know what address to tag the references with; it gets 
relocated by gs_base down into proper kernel space.  The linker can 
stash the initialized reference copy at any address (LMA) which can be 
different from what it will be used at (VMA); that is not an issue.

To use %rip references, though, which are more efficient, you probably 
want to use offsets that are just below .text (at -2 GB); presumably
-2 GB-[max size of percpu section].  Again, however, no CPU actually 
needs to have its data stashed in that particular location; it's just an 
offset.

	-hpa
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/