Re: [Qemu-devel] [RFC v3 46/56] accel/tcg: convert to cpu_interrupt_request

[Emilio G. Cota]

On Tue, Oct 23, 2018 at 03:17:11 +0100, Richard Henderson wrote:
> On 10/23/18 12:50 AM, Emilio G. Cota wrote:
> > 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.
> 
> Why not?  That's even cheaper.
> 
> > Given that the CPU lock is uncontended (so it's cheap to
> > acquire) ...
> 
> It still requires at minimum a "lock xchg" (or equivalent on non-x86), which
> isn't free -- think 50-ish cycles minimum just for that one insn, plus call
> overhead.

OK, I changed the first read to atomic_read (changing all the other
writers to atomic_set, but thanks to the helpers it's just very
few of them), and then I'm holding both the BQL + cpu->lock throughout.

Thanks,

                Emilio