Re: [Qemu-devel] [PULL 07/11] cpu-exec: don't overwrite exception_index

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Peter Maydell [mailto:peter.maydell@linaro.org]
> On 9 January 2018 at 13:21, Pavel Dovgalyuk <dovgaluk@ispras.ru> wrote:
> > I tried to get some logs with the following code.
> > It prints that there was an exception 5 and it was overwritten by the standard code.
> > Fixed code prevents this overwrite.
> >
> > I guess that one of the following is true:
> >  - unfixed version misses some exceptions
> >  - fixed version processes some exceptions twice (e.g., when there is no clear exception)
> >
> > diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> > index 280200f..fa810f7 100644
> > --- a/accel/tcg/cpu-exec.c
> > +++ b/accel/tcg/cpu-exec.c
> > @@ -605,6 +605,8 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
> >      /* Finally, check if we need to exit to the main loop.  */
> >      if (unlikely(atomic_read(&cpu->exit_request)
> >          || (use_icount && cpu->icount_decr.u16.low + cpu->icount_extra == 0)))
> > +        if (cpu->exception_index != -1 && cpu->exception_index != EXCP_INTERRUP
> > +            qemu_log("overwriting excp_index %x\n", cpu->exception_index);
> >          atomic_set(&cpu->exit_request, 0);
> >          cpu->exception_index = EXCP_INTERRUPT;
> >          return true;
> 
> This looks like it's just working around whatever is going on
> (why should EXCP_INTERRUPT be special?). What we need to do is
> find out what's actually happening here...

The failure cause is in incorrect interrupt processing.
When ARM processes hardware interrupt in arm_cpu_exec_interrupt(),
it executes cs->exception_index = excp_idx;

This assumes, that the exception will be processed later.
But it is processed immediately by calling cc->do_interrupt(cs);
instead of leaving this job to cpu_exec.

I guess these calls should be removed to match the cpu_exec execution pattern.

Pavel Dovgalyuk