Percpu data in a vsyscall page

Percpu data in a vsyscall page

[Vojtech Pavlik]

Hi!

I'm working on a RDTSCP support on x86-64, and for that, I'll need
per-cpu time offset table in a vsyscall page. I saw the percpu.h header,
and thought - "Hey, I could use that!", but I think I really can't.

The data need to be in a vsyscall page, which is mapped to userspace via
linker magic, and the percpu stuff uses a different mapping.

I'm using a simple array instead, because the address will be different
in a vsyscall from the one the kernel sees anyway, and RDTSCP will give
me an index to that array atomically.

Is there any problem with that approach? Is there any reason using
percpu.h would be better? 

-- 
Vojtech Pavlik
SuSE Labs, SuSE CR

Re: Percpu data in a vsyscall page

[Rusty Russell]

On Wed, 2005-11-02 at 11:54 +0100, Vojtech Pavlik wrote:
> Hi!
> 
> I'm working on a RDTSCP support on x86-64, and for that, I'll need
> per-cpu time offset table in a vsyscall page. I saw the percpu.h header,
> and thought - "Hey, I could use that!", but I think I really can't.
> 
> The data need to be in a vsyscall page, which is mapped to userspace via
> linker magic, and the percpu stuff uses a different mapping.

Yes.  The percpu stuff uses a single add to find the local value of a
variable.  This per-cpu offset can be kept in a register (I think
Sparc64 already does this) making it v. fast.

You're going to have to stick with an array I think.

Sorry,
Rusty.
-- 
A bad analogy is like a leaky screwdriver -- Richard Braakman