Re: [RFC 00/15] x86_64: Optimize percpu accesses

["H. Peter Anvin" <hpa@zytor.com>]

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