Re: [PATCH] powerpc: New feature - HWCAP/HWCAP2 bits in the TCB

[Steven Munroe <munroesj at linux dot vnet dot ibm dot com>]

On Tue, 2015-06-09 at 16:48 +0100, Szabolcs Nagy wrote:
> 
> On 09/06/15 16:06, Steven Munroe wrote:
> > On Tue, 2015-06-09 at 15:47 +0100, Szabolcs Nagy wrote:
> >> i assume this is for multi-versioning.
> > 
> > The intent is for the compiler to implement the equivalent of
> > __builtin_cpu_supports("feature"). X86 has the cpuid instruction, POWER
> > is RISC so we use the HWCAP. The trick to access the HWCAP[2]
> > efficiently as getauxv and scanning the auxv is too slow for inline
> > optimizations.
> 
> i think getauxv is not usable by the compiler anyway,
> it's not a standard api.
> 
> >> i dont see how the compiler can generate code to access the
> >> hwcap bits currently (without making assumptions about libc
> >> interfaces).
> >>
> > These offset will become a durable part the PowerPC 64-bit ELF V2 ABI.
> > 
> > The TCB offsets are already fixed and can not change from release to
> > release.
> 
> hard coded arch specific tcb offsets make sure that
> targets need different tcb layout which means more
> target specific maintainance instead of common c code.
> 
> >> if hwcap is useful abi between compiler and libc
> >> then why is this done in a powerpc specific way?
> > 
> > Other platform are free use this technique.
> 
> i think this is not a sustainable approach for
> compiler abi extensions.
> 
> (it means juggling with magic offsets on the order
> of compilers * libcs * targets).
> 
> unfortunately accessing the ssp canary is already
> broken this way, i'm not sure what's a better abi,
> but it's probably worth thinking about one before
> the tcb code gets too messy.
> 

I have thought about it.

Based on my detailed knowledge of the PowerISA and PowerPC ABIs this the
simplest and fastest solution.