Re: [RFC PATCH] semihosting: suspend recieving CPU when blocked (HACK, WIP)

["Alex Bennée" <alex.bennee@linaro.org>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 17/12/19 14:42, Alex Bennée wrote:
>>> Why do you need to set exception_index to something other than -1 (using
>>> cpu_loop_exit_noexc for example)?
>> If there is no exception to process we won't exit the main loop which we
>> need to do if we want to wait until there is data to read.
>
> Okay.
>
>>> Using ->stop here is a bit weird, since ->stop is usually related to
>>> pause_all_vcpus.
>> 
>> Arguably we could come up with a better API to cpu.c but this allows us
>> to use cpu_resume(c->sleeping_cpu) when waking up rather than hand
>> rolling our own wake-up mechanism.
>
> But we already have the right wake-up mechanism, which is
> cpu->halted/cpu_has_work.

cpu_has_work is a guest function though and semihosting_console is a
common hw module. It can't peek into the guests internal state. This all
comes back to cpu_thread_is_idle anyway in making our decision about if
we do or do not sleep on the halt_cond.

> That also makes it possible to just use
> EXCP_HALTED instead of adding a new EXCP_BLOCKED.

We can certainly use EXCP_HALTED but maybe come up with a common way of
entering the state? There seems to be a combination of messing around
with special interrupts and direct poking of cs->halted = 1 while
setting the exception. Maybe this could finally clear up the #if
defined(TARGET_I386) hacking in cpus.c?

>
> Paolo


-- 
Alex Bennée