Messages in this thread | | | Date | Wed, 09 Jul 2008 17:12:14 -0400 | From | "H. Peter Anvin" <> | Subject | Re: [RFC 00/15] x86_64: Optimize percpu accesses |
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Jeremy Fitzhardinge wrote: > H. Peter Anvin wrote: >> 1. it means pda references are invalid if their offsets are ever more >> than CONFIG_PHYSICAL_BASE (which I do not think is likely, but still...) > > Why? > > As an aside, could we solve the problems by making CONFIG_PHYSICAL_BASE > 0 - putting the percpu variables as the first thing in the kernel - and > relocating on load? That would avoid having to make a special PT_LOAD > segment at 0. Hm, would that result in the pda and the boot params > getting mushed together? >
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.
Either way, I really suspect that the right thing to do is to use negative offsets, with the possible exception of a handful of things (40 bytes or less, perhaps like current) which can get small positive offsets and end up in the "super hot" cacheline.
The sucky part is that I don't believe GNU ld has native support for a "hanging down" section (one which has a fixed endpoint rather than a starting point), so it requires extra magic around the link (or finding some way to do it with linker script functions.) Let me see if I can cook up something in linker script that would actually work.
-hpa
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