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

[Paolo Bonzini <pbonzini@redhat.com>]

On 17/12/19 15:18, Alex Bennée wrote:
> 
> 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.

semihosting_console only needs to something like
cpu_interrupt(cpu->stopped_cpu, CPU_INTERRUPT_SEMIHOST).  (By the way,
the stopped_cpu should probably be a list to mimic the condition
variable---for example a GSList).

> 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?

If you're talking accel/tcg/cpu-exec.c, that's different; the issue
there is that x86 has a kind of warm reset pin that is not equivalent to
cpu_reset.  Removing that would only entail adding a new member function
to CPUClass.

Paolo