Re: [Qemu-arm] [Qemu-devel] [PATCH 02/15] target/arm: Don't trap WFI/WFE for M profile

["Edgar E. Iglesias" <edgar.iglesias@gmail.com>]

On Thu, Aug 03, 2017 at 01:28:28PM -0700, Richard Henderson wrote:
> On 08/02/2017 09:43 AM, Peter Maydell wrote:
> > M profile cores can never trap on WFI or WFE instructions. Check for
> > M profile in check_wfx_trap() to ensure this.
> > 
> > The existing code will do the right thing for v7M cores because
> > the hcr_el2 and scr_el3 registers will be all-zeroes and so we
> > won't attempt to trap, but when we start setting ARM_FEATURE_V8
> > for v8M cores the v8A handling of SCTLR.nTWE and .nTWI will not
> > give the right results.
> > 
> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> > ---
> >  target/arm/op_helper.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> 
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> 
> While looking at this, I think there's an error in helper_wfi.  The early exit
> for cpu_has_work should happen after the exception check.

I don't have the spec at hand but IIRC the trap should only happen
if the processor would have entered the low-power state (i.e if
there's no work).

A comment in the code would probably be good...

Cheers,
Edgar