Re: [Qemu-devel] [RFC PATCH] target/arm: ensure eret exits the run-loop

[Richard Henderson]

On 07/07/2017 08:29 AM, Alex Bennée wrote:
> > Naming all of these different exit conditions is certainly
> > non-trivial.
>
> Given the variation of usage this is something that should probably be
> done after Lluís common run loop goes in and we can beef up the
> semantics of the various exit conditions.

Definitely.

> One thing I have noticed in the ARM translator is DISAS_UPDATE does a:
>
>      gen_a64_set_pc_im(dc->pc);
>
> I think this is to deal with handling exceptions either side of various
> instructions. Am I right in thinking this is superfluous now as we can
> derive the PC from the translated code address?

Yes and no.

We have typically distinguished between two kinds of exceptions: those that are 
dynamic (fp state, page permissions) and those that are static (illegal opcodes).

For the dynamic, we used to pessimistically save state, but now use the 
unwinder to restore it.  The unwinding is expensive but is used infrequently 
(especially compared to the number of load/store insns executed).

For the static, we know the exception must be raised, and we know the state 
that must be saved.  By doing that, we save the expense of the unwinding.

So, for the static case you're talking about, we could get the PC (and other 
state) back, and remove the explicit stores, but we shouldn't.


r~