Re: [Qemu-arm] [PATCH] fix WFI/WFE length in syndrome register

[Stefano Stabellini]

On Fri, 20 Oct 2017, Peter Maydell wrote:
> On 19 October 2017 at 21:56, Stefano Stabellini <address@hidden> wrote:
> > On Thu, 19 Oct 2017, Peter Maydell wrote:
> >> On 18 October 2017 at 23:03, Stefano Stabellini <address@hidden> wrote:
> >> > WFI/E are 4 bytes long: set ARM_EL_IL_SHIFT in the syndrome.
> >> >
> >> > Signed-off-by: Stefano Stabellini <address@hidden>
> >> >
> >> > diff --git a/target/arm/internals.h b/target/arm/internals.h
> >> > index 1f6efef..cf8c966 100644
> >> > --- a/target/arm/internals.h
> >> > +++ b/target/arm/internals.h
> >> > @@ -398,6 +398,7 @@ static inline uint32_t syn_breakpoint(int same_el)
> >> >  static inline uint32_t syn_wfx(int cv, int cond, int ti)
> >> >  {
> >> >      return (EC_WFX_TRAP << ARM_EL_EC_SHIFT) |
> >> > +           (1 << ARM_EL_IL_SHIFT) |
> >> >             (cv << 24) | (cond << 20) | ti;
> >> >  }
> >>
> >> Hmm. What we do now is definitely wrong, but WFI and WFE can be 2 bytes:
> >> there is a T1 Thumb encoding that is 2 bytes.
> >>
> >> HELPER(wfi) doesn't get that right, though:
> >>     if (target_el) {
> >>         env->pc -= 4;
> >>         raise_exception(env, EXCP_UDEF, syn_wfx(1, 0xe, 0), target_el);
> >>     }
> >>
> >> So I think that HELPER(wfi) needs to be passed an extra
> >> parameter is_16bit, which it can then use both in its adjustment
> >> of env->pc and to pass as an extra parameter to syn_wfx(),
> >> which is then syn_wfx(int cv, int cond, int ti, bool is_16bit).
> >>
> >> (In theory HELPER(wfe) should also be passed is_16bit, but
> >> since it doesn't currently ever raise an exception it
> >> doesn't matter.)
> >
> > Wouldn't it be better to just check on env->thumb like
> > HELPER(cpsr_write_eret) for example?
> 
> > diff --git a/target/arm/op_helper.c b/target/arm/op_helper.c
> > index 3914145..ea16c9a 100644
> > --- a/target/arm/op_helper.c
> > +++ b/target/arm/op_helper.c
> > @@ -476,8 +476,8 @@ void HELPER(wfi)(CPUARMState *env)
> >      }
> >
> >      if (target_el) {
> > -        env->pc -= 4;
> > -        raise_exception(env, EXCP_UDEF, syn_wfx(1, 0xe, 0), target_el);
> > +        env->pc -= env->thumb ? 2 : 4;
> > +        raise_exception(env, EXCP_UDEF, syn_wfx(1, 0xe, 0, env->thumb), 
> > target_el);
> >      }
> >
> >      cs->exception_index = EXCP_HLT;
> 
> This doesn't work, because there is also a 32 bit Thumb
> encoding of WFI. env->thumb says "we are in thumb mode",
> which isn't the same as "this insn is 16 bits".

OK, I understand your suggestion now. I'll send a new patch.