Messages in this thread | | | Date | Tue, 20 Nov 2007 12:43:10 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [rfc 08/45] cpu alloc: x86 support |
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Andi Kleen wrote: > On Tuesday 20 November 2007 04:50, Christoph Lameter wrote: >> On Tue, 20 Nov 2007, Andi Kleen wrote: >>> I might be pointing out the obvious, but on x86-64 there is definitely >>> not 256TB of VM available for this. >> Well maybe in the future. > > That would either require more than 4 levels or larger pages > in page tables. > >> One of the issues that I ran into is that I had to place the cpu area >> in between to make the offsets link right. > > Above -2GB, otherwise you cannot address them >
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.
Obviously the offsets *within* the percpu area has to be in range (±2 GB per cpu for absolute offsets, slightly smaller for %rip-based addressing -- obviously judicious use of an offset for gs_base is essential in the latter case).
Thus you want the percpu areas below -2 GB where they don't interfere with modules or any other precious address space.
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