Re: [PATCH v1 02/21] target/alpha: add BQL to do_interrupt and cpu_exec_interrupt

[Robert Foley]

On Wed, 5 Aug 2020 at 15:18, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> On 8/5/20 11:12 AM, Robert Foley wrote:
> > @@ -299,8 +299,12 @@ void alpha_cpu_do_interrupt(CPUState *cs)
> >  {
> >      AlphaCPU *cpu = ALPHA_CPU(cs);
> >      CPUAlphaState *env = &cpu->env;
> > -    int i = cs->exception_index;
> > -
> > +    int i;
> > +    bool bql = !qemu_mutex_iothread_locked();
> > +    if (bql) {
> > +        qemu_mutex_lock_iothread();
> > +    }
>
> Why does this patch for alpha need to check qemu_mutex_iothread_locked and the
> next patch for arm does not?
>

In alpha (and arm) the do_interrupt function can be called separately or by
cpu_exec_interrupt.  In the case where do_interrupt gets called separately
it needs to take the BQL (bql == true).
In the case where cpu_exec_interrupt is holding the BQL, and calls do_interrupt,
do_interrupt needs to check qemu_mutex_iothread_locked, and in this case not get
the lock (bql == false).

The next patch for arm, checks qemu_mutex_iothread_locked in its do_interrupt
function, but not in its cpu_exec_interrupt function, the same pattern
as for alpha.

Thanks & Regards,
-Rob

>
> r~