Re: [RFC, patch] i386: vgetcpu(), take 2

[Andi Kleen <ak@suse.de>]

> Would sgdt not be sufficient?  I agree that we will have to end up
> giving RO access to user for the gdt page.

I meant exporting the GDT page

> I agree that we should not overload a single call (though cpu, package
> and node numbers do belong in one category IMO).  We can have multiple
> calls if that is required as long as there is an efficient mechanism to
> provide that information.

The current mechanism doesn't scale to much more calls, but I guess
i'll have to do a vDSO sooner or later.
 
> Why maintain that extra logic in user space when kernel can easily give
> that information.

It already does.
 
> > I've been pondering to put some more information about that
> > in the ELF aux vector, but exporting might work too. I suppose
> > exporting would require the vDSO first to give a sane interface.
> > 
> Can you please tell me what more information you are thinking of putting
> in aux vector?

One proposal (not fully fleshed out was) number of siblings / sockets / nodes 
I don't think bitmaps would work well there (and if someone really needs
those they can read cpuinfo again) 

This is mostly for OpenMP and tuning of a few functions (e.g. on AMD
the memory latencies varies with the number of nodes so some functions
can be tuned in different ways based on that) 

> You are absolutely right that the mechanism I'm proposing makes sense
> only if we have more fields AND if any of those fields are dynamically
> changing.  But this is a generic mechanism that could be extended to
> share any user visible information in efficient way.  Once we have this
> in place then information like whole cpuinfo, percpu interrupts etc. can
> be retrieved easily.

The problem with exposing too much is that it might be a nightmare
to guarantee a stable ABI for this. At least it would
constrain the kernel internally. Probably less is better here. 

Also I'm still not sure why user space should care about interrupts?

-Andi