[PATCH] powerpc64/linux: Fix 64-bit process detection on embedded

[Dmitry V. Levin]

On Wed, Feb 19, 2014 at 07:10:53PM -0600, James Yang wrote:
> On Thu, 20 Feb 2014, Dmitry V. Levin wrote:
> 
> > On Tue, Feb 18, 2014 at 03:32:43PM -0600, James Yang wrote:
> > > * syscall.c: Fix 64-bit process detection on embedded powerpc
> > > 
> > > Signed-off-by: James Yang <james.yang at freescale.com>
> > > ---
> > >  syscall.c |    5 +++--
> > >  1 files changed, 3 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/syscall.c b/syscall.c
> > > index 3477dcd..8d47e7e 100644
> > > --- a/syscall.c
> > > +++ b/syscall.c
> > > @@ -1222,8 +1222,9 @@ get_scno(struct tcb *tcp)
> > >  	int currpers;
> > >  
> > >  	/* Check for 64/32 bit mode. */
> > > -	/* SF is bit 0 of MSR */
> > > -	if ((ppc_regs.msr >> 63) & 1)
> > > +	/* SF is bit 0 of MSR (a 64-bit register in Server) */
> > > +	/* CM is bit 0 of MSR (a 32-bit register in Embedded) */
> > > +	if (((ppc_regs.msr >> 63) & 1) || ((ppc_regs.msr >> 31) & 1))
> > >  		currpers = 0;
> > >  	else
> > >  		currpers = 1;
> > 
> > In case of 64-bit MSR, that would be a test for 32th bit.
> > I'm not quite familiar with ppc64, is there any risk for a false positive
> > in this case?
> 
> When tested on a POWER7 running Linux, MSR bit 32 is clear.  So 
> empircally, it is not in use.
> 
> It is documented as part of a reserved field according to Power ISA 
> Book III-S so it should read as 0 unless there's an implementation- 
> specific use of the bit.  No documentation I have found describes the 
> use of it as other than reserved.

OK, let's assume it's safe to do the check this way.

There seems to be a shorter equivalent check for these two bits:
	ppc_regs.msr & 0x8000000080000000

Which of them would you prefer, the longer one or the shorter one?


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