Re: [Qemu-devel] [PATCH 05/10] cpu-exec: elide more icount code if CONFIG_USER_ONLY

[Paolo Bonzini]

On 28/08/2015 16:56, Peter Maydell wrote:
> What's the rationale for this? Mostly we prefer not to
> add ifdefs in code if we can get away with compiling it for
> both cases, even if the resulting code isn't used.

True.  The rationale is three-fold:

1) It makes sense to abort if TB_EXIT_ICOUNT_EXPIRED is returned from
user-mode emulation TCG code.  Then you need to either leave the
unreachable code after abort() or #ifdef cpu_exec_nocache out.

2) Dually, cpu_exec_nocache's future locking requirements (take tb_lock,
mostly) are not yet respected as of this patch.  Marking the function as
#ifdef CONFIG_SOFTMMU explains why.  Once you do that, you have to
abort() either in cpu_exec_nocache or in the caller.


Reason (2) is stronger for me.  You mentioned a few times your hope that
MTTCG would lead to more correct and more documented thread support in
user-mode emulation, and I think this patch does improve the
documentation slightly.

Paolo