Messages in this thread | | | Date | Wed, 09 Jul 2008 18:23:45 -0400 | From | "H. Peter Anvin" <> | Subject | Re: [RFC 00/15] x86_64: Optimize percpu accesses |
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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
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