On Sun, Oct 21, 2018 at 14:34:25 +0100, Richard Henderson wrote:
> On 10/19/18 2:06 AM, Emilio G. Cota wrote:
> > @@ -540,16 +540,16 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
> > */
> > atomic_mb_set(&cpu->icount_decr.u16.high, 0);
> >
> > - if (unlikely(atomic_read(&cpu->interrupt_request))) {
> > + if (unlikely(cpu_interrupt_request(cpu))) {
> > int interrupt_request;
> > qemu_mutex_lock_iothread();
> > - interrupt_request = cpu->interrupt_request;
> > + interrupt_request = cpu_interrupt_request(cpu);
> > if (unlikely(cpu->singlestep_enabled & SSTEP_NOIRQ)) {
> > /* Mask out external interrupts for this step. */
> > interrupt_request &= ~CPU_INTERRUPT_SSTEP_MASK;
> > }
> > if (interrupt_request & CPU_INTERRUPT_DEBUG) {
> > - cpu->interrupt_request &= ~CPU_INTERRUPT_DEBUG;
> > + cpu_reset_interrupt(cpu, CPU_INTERRUPT_DEBUG);
> > cpu->exception_index = EXCP_DEBUG;
> > qemu_mutex_unlock_iothread();
> > return true;
>
> Multiple calls.
I'd rather keep it as is.
The first read takes the lock, and that has to stay unless
we want to use atomic_set on interrupt_request everywhere.
The second read happens after the BQL has been acquired;
note that to avoid deadlock we never acquire the BQL after
a CPU lock; the second (locked) read thus has to stay.
Subsequent accesses are all via cpu_reset_interrupt.
If we wanted to avoid reacquiring the lock, we'd have
to explicitly acquire the lock before the 2nd read,
and add unlocks everywhere (like the many qemu_mutex_unlock_iothread
calls), which would be ugly. But we'd also have to be careful
not to longjmp with the CPU mutex held, so we'd have to
unlock/lock around cc->cpu_exec_interrupt.
Given that the CPU lock is uncontended (so it's cheap to
acquire) and that the cases where we call cpu_reset_interrupt
are not that frequent (CPU_INTERRUPT_{DEBUG,HALT,EXITTB}),
I'd rather just keep the patch as is.
Thanks,
Emilio
