Messages in this thread | | | Date | Wed, 1 Apr 2009 10:32:57 +0200 | From | Martin Schwidefsky <> | Subject | Re: [GIT RFC] percpu: use dynamic percpu allocator as the default percpu allocator |
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On Wed, 01 Apr 2009 17:17:33 +0900 Tejun Heo <tj@kernel.org> wrote:
> Martin Schwidefsky wrote: > > Is the goal to use the same access macros for both dynamically and > > statically allocated percpu variables? That would make the proposed > > solution impractical. > > Yeah, it's one of the goals so that we don't have to have two sets of > APIs (e.g. the fast percpu_*() accessors).
Uh-oh..
> > The "X" constraint trick we used so far tells the compiler to pass the > > argument verbatim to the assembler. The assembler knows how to deal > > with symbol@GOTENT. If we pass a gcc variable or even a more general > > term via an "X" constraint the assembler gets <something C-ish>@GOTENT. > > It is not possible to parse this in the assembler. To do what you want > > to achieve would mean to avoid using the "X" constraint. Which means > > that we cannot use the GOTENT trick anymore. If we let the compiler > > resolve the address for a static per cpu variable we end up with the > > larl problem. Ouch! > > > > So far the SHIFT_PERCPU_PTR/SHIFT_PERCPU_VAR is the only solution > > I have found for s390 and the dynamic percpu allocator. I'll keep > > looking for a better solution but I am not optimistic. > > What does the assembler do when it gets GOTENT? GOTENT sounds like > global offset table entry, so does it make the assembler emit an entry > in GOT and then get the address indirectly?
Yes, @GOTENT is a relocation against the GOT slot that contains the address of the symbol. It is a special version of @GOT that uses larl to locate the got slot directly without the need of a got base pointer.
The code sequence with @GOT:
larl %r12,_GLOBAL_OFFSET_TABLE_ lg %r1,symbol@GOT(%r12)
is equivalent to:
larl %r1,symbol@GOTENT lg %r1,0(%r1)
The advantage of the second code sequence is that it need a single register and the size of the GOT is not limited to 4K as in the first example (the offset in an RX format instruction is limited to 12 bits - but that is probably something you don't want to know ;-).
-- blue skies, Martin.
"Reality continues to ruin my life." - Calvin.
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