vDSO preliminary implementation

[Linas Vepstas]

On Sat, Sep 11, 2004 at 10:43:38AM +1000, Benjamin Herrenschmidt was heard to remark:
> > > 
> > > Here's a first shot at implementing a vDSO for ppc32/ppc64. This is definitely
> > 
> > What's vDSO ?  Google was amazingly unhelpful in figuring this out.
> 
> virtual .so, that is a library mapped by the kernel in userspace

Let me re-phrase that: what's it good for?  Is this a mechanism for 
sharing the text segment of a library between all users?

Ye olde AIX had this feature; I've never thought about whether Linux
does this or not; shared libs were loaded so that the text segment
of a library appeared only once in 'real' memory, and was thus shared
by the various apps.  I'm not sure, I think in AIX even the "ptes" were
shared too: the text was always loaded into the same segment (segment 0
iirc), so you wouldn't have tlb misses on things like libc. 

I've never thought about how Linux loads libraries, so excuse me on this
newbie-sounding question.  How does Linux load .so's today?  Is there
one copy per process, or are they shared?

--linas