Re: RFC: partially available registers

[Pedro Alves <pedro@codesourcery.com>]

On Friday 22 July 2011 20:10:23, Pedro Alves wrote:
> On Friday 22 July 2011 19:55:31, Tom Tromey wrote:
> > >>>>> "Pedro" == Pedro Alves <pedro@codesourcery.com> writes:
> > 
> > Pedro> Ah, that's it then.  I was curious to know why were the
> > Pedro> upper parts of the ymm unavailable.
> > 
> > amd64_linux_fetch_inferior_registers calls ptrace(PTRACE_GETREGSET)
> > to fetch the registers.  Then it passes this to amd64_supply_xsave,
> > which calls i387_supply_xsave.  This function then decodes the "XCR0"
> > flag and determines that the upper parts were not supplied by the
> > kernel; that is, we take the true branch here:
> > 
> > 	  if ((clear_bv & I386_XSTATE_AVX))
> > 	    p = NULL;
> > 	  else
> > 	    p = regs;
> 
> Ah, thanks.  With a bit more context:
> 
>     case avxh:
>       if ((clear_bv & I386_XSTATE_AVX))
>         p = NULL;
>       else
>         p = XSAVE_AVXH_ADDR (tdep, regs, regnum);
>       regcache_raw_supply (regcache, regnum, p);
>       return;
> 
> regcache_raw_supply with p=NULL means the register
> is unavailable.  But before the <unavailable> stuff,
> it meant "supply the register as 0".  I seem to remember
> discussing this AVX stuff with H.J., and coming to the
> conclusion that what want is really 0, but maybe not.

Found it:

<http://sourceware.org/ml/gdb-patches/2010-10/msg00268.html>

(and follow up)

> gdbserver is explicitly zeroing in this case, instead
> of returning unavailable, see
> gdbserver/i387-fp.c:i387_xsave_to_cache.  
> 
> What does it really mean when you have an AVX
> capable machine, but I386_XSTATE_AVX is clear?
> 
> Whatever the answer, we need to fix one of native
> gdb or gdbserver for consistency.

-- 
Pedro Alves