Re: [Qemu-devel] [RFC/RFT PATCH 0/7] cpu-exec: simplify cpu_exec and remove some icount special cases

[Pavel Dovgalyuk]

> From: Paolo Bonzini [mailto:address@hidden On Behalf Of Paolo Bonzini
> On 31/01/2017 01:05, Pavel Dovgalyuk wrote:
> > Hi, Paolo!
> >
> > Thanks for refactoring.
> > I tested these patches with icount record/replay on i386 machine.
> > It works, but the following changes should be applied.
> > I also removed call to replay_has_interrupt, because now it is not needed 
> > here.
> > It seems, that this call is an artifact of an older record/replay revision.
> >
> > diff --git a/cpu-exec.c b/cpu-exec.c
> > index 3838eb8..5cef8bc 100644
> > --- a/cpu-exec.c
> > +++ b/cpu-exec.c
> > @@ -519,7 +519,8 @@ 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) || 
> > replay_has_interrupt())) {
> > +    if (unlikely(atomic_read(&cpu->exit_request)
> > +        || (use_icount && cpu->icount_decr.u16.low + cpu->icount_extra == 
> > 0))) {
> >          atomic_set(&cpu->exit_request, 0);
> >          cpu->exception_index = EXCP_INTERRUPT;
> >          return true;
> 
> So is this needed to avoid exceptions in tb_find?  Please add a comment
> about this 

This code comes from my last patch, that was not applied.
Here is the comment:

It adds check to break cpu loop when icount expires without
setting the TB_EXIT_ICOUNT_EXPIRED flag. It happens when there is no
available translated blocks and all instructions were executed.
In icount replay mode unnecessary tb_find will be called (which may
cause an exception) and execution will be non-deterministic.

> and check if you can also replace:
> 
>       atomic_set(&cpu->exit_request, 1);
> 
> in cpu_loop_exec_tb with
> 
>       cpu->icount_decr.u16.low = 0;
> 
> ?
> 

This line is not needed at all, because the following code decrements
icount automatically.

        if (insns_left > 0) {
            cpu_exec_nocache(cpu, insns_left, tb, false);
        }

Pavel Dovgalyuk