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Re: [PATCH] PowerPC64: Add __private_ss field to TCB header

On Tue, 2013-11-12 at 22:19 +0100, Ulrich Weigand wrote:
> Hello,
> the TCB header on Intel contains a field __private_ss that is used
> to efficiently implement the -fsplit-stack GCC feature.
> In order to prepare for a possible future implementation of that
> feature on powerpc64, we'd like to reserve a similar field in
> the TCB header as well.  (It would be good if this went in with
> or before the ELFv2 patches to ensure that this field will be
> available always in the ELFv2 environment.)
> The field needs to be added at the front of tcbhead_t structure
> to avoid changing the ABI; see the recent discussion when adding
> the EBB fields.
> Tested on powerpc64-linux and powerpc64le-linux.
> OK for mainline?
I have to remind myself each time how this sould work ...

"The tp offset allows for efficient addressing of the TCB and up to
4K-16 of other thread library information."


/* The following assumes that TP (R2 or R13) points to the end of the
   TCB + 0x7000 (per the ABI).  This implies that TCB address is
   TP - 0x7000.  As we define TLS_DTV_AT_TP we can
   assume that the pthread struct is allocated immediately ahead of the
   TCB.  This implies that the pthread_descr address is
   TP - (TLS_PRE_TCB_SIZE + 0x7000).  */

The key is that new fields are added above previous fields so to
maintain upward compatibility release to release.

Adhemerval is that correct?

> Bye,
> Ulrich
> nptl/ChangeLog:
> 2013-11-12  Ulrich Weigand  <>
> 	* sysdeps/powerpc/tls.h (tcbhead_t): Add __private_ss field.
> Index: glibc/nptl/sysdeps/powerpc/tls.h
> ===================================================================
> --- glibc.orig/nptl/sysdeps/powerpc/tls.h
> +++ glibc/nptl/sysdeps/powerpc/tls.h
> @@ -61,6 +61,8 @@ typedef union dtv
>     are private.  */
>  typedef struct
>  {
> +  /* GCC split stack support.  */
> +  void *__private_ss;
>    /* Reservation for the Event-Based Branching ABI.  */
>    uintptr_t ebb_handler;
>    uintptr_t ebb_ctx_pointer;

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