[mips-tls] Revised versions of MIPS TLS ABI specification

[Daniel Jacobowitz]

On Thu, Nov 04, 2004 at 06:00:53PM +0000, Nigel Stephens wrote:
> Daniel Jacobowitz wrote:
> 
> >>The following issue does need resolution:
> >>
> >>1. Should we extend the 32K Local Dynamic Model to 64K by using a biased 
> >>offset?
> >>
> >>Daniel and I thought this would be more trouble than it's worth; Thiemo 
> >>thinks otherwise.  Are there any other opinions?
> >>   
> >>
> >
> >I have since learned that PowerPC already does this:
> >
> >| The PowerPC32 TLS ABI is similar to the PowerPC64 model.  The
> >| thread-local storage data structures follow variant I.  The TCB is 8
> >| bytes, with the first 4 bytes containing the pointer to the dynamic
> >| thread vector. tlsoffset calculations and definition of __tls_get_addr
> >| are identical to PowerPC64.  r2 is the thread pointer, and points
> >| 0x7000 past the end of the thread control block.  Dynamic thread vector
> >| pointers point 0x8000 past the start of each TLS block.  (*) This
> >| allows the first 64K of each block to be addressed from a dtv pointer
> >| using fewer machine instructions.  The tp offset allows for efficient
> >| addressing of the TCB and up to 4K-8 of other thread library
> >| information.
> >| 
> >| (*) For implementation reasons the actual value stored in dtv may point
> >| to the start of a block, however values returned by accessor functions
> >| will be offset by 0x8000.
> > 
> >
> 
> Hi Daniel
> 
> I'd be interested to know where that quoted passage comes from - is it a 
> public document, I can't find it with Google?

Take a look at
  ftp://ftp.linuxppc64.org/pub/people/amodra

(from memory; I hope I got the path right.)

> >Shall we use this model? 
> 
> 
> It does sounds like we could use the same trick. But I'd like to 
> understand it better. I thought that the problem was that in variant I 
> the size of the TCB was indeterminate, so the static linker couldn't 
> insert optimised references to initial-exec TLS data as fixed offsets 
> from the thread pointer. But the implication of the above is that the 
> TCB does have a known, fixed size, so the a fixed offset can now  be 
> inserted by the linker, as for variant II. Have I got that right?

No.  This is a variant I model, which means that the size and layout of
the TCB are known; it's in variant II that the TCB is indeterminate. In
variant I the TLS areas have a positive offset from the TCB, which
starts with a pointer to the DTV; in variant II they have a negative
offset from the TCB, which has indeterminate contents.  In either case
the static linker can resolve IE/LE references that bind to the local
executable.

Normally in variant I the thread pointer points to the beginning of the
TCB, i.e. a known distance before the start of the first TLS block.  In
this case the thread pointer points to 0x7000 past the _end_ of the
first TLS block - but that's just an implementation detail.

-- 
Daniel Jacobowitz