Re: [PATCH] powerpc: 32 bit getcpu VDSO function uses 64 bit instructions

[Segher Boessenkool <segher@kernel.crashing.org>]

On Thu, Nov 27, 2014 at 11:41:40AM -0600, Peter Bergner wrote:
> On Thu, 2014-11-27 at 10:08 -0600, Segher Boessenkool wrote:
> > On Wed, Nov 26, 2014 at 05:50:27PM -0600, Peter Bergner wrote:
> > > Nope, you don't get a SIGILL when executing 64-bit instructions in
> > > 32-bit mode, so it'll happily just execute the instruction, doing
> > > a full 64-bit compare.  I'm guessing that the upper 32-bits of both
> > > r3 and r4 contain zeros, so we're probably just getting lucky.
> > 
> > You will get a SIGILL if you run on 32-bit hardware.
> 
> Ha, I completely forgot about 32-bit hardware.  Anyway, I looked
> at the ISA, and cmpdi and cmpwi are just extended mnemonics for
> cmpi, with cmpdi setting the L field to 1.  Probably on 32-bit
> hardware, the hardware is just ignoring the L bit being set and
> doing a cmpwi for us???

Huh.  Yes, maybe some implementations do that.

The good news is that those then compute the correct thing ;-)

Can QEMU help catch such bugs more reliably?


Segher