Re: [PATCH 2/2] accel/tcg: interrupt/exception handling uses bql_interrupt flag

[Robert Foley <robert.foley@linaro.org>]

On Fri, 31 Jul 2020 at 14:02, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 31/07/20 14:51, Robert Foley wrote:
> > This change removes the implied BQL from the cpu_handle_interrupt,
> > and cpu_handle_exception paths. We can now select per-arch if
> > the BQL is needed or not by using the bql_interrupt flag.
> > By default, the core code holds the BQL.
> > One benefit of this change is that it leaves it up to the arch
> > to make the change to remove BQL when it makes sense.
> >
> > Signed-off-by: Robert Foley <robert.foley@linaro.org>
>
> No, please just modify all implementation to do lock/unlock.  It's a
> simpler patch than this on.

Sure, we will update the patch based on this.

To clarify, the suggestion here is to remove the bql_interrupt flag
that we added and change all the per-arch interrupt callback code to
do the lock/unlock of the BQL?  So for example change
x86_cpu_exec_interrupt, and arm_cpu_exec_interrupt, etc to lock/unlock BQL?

Thanks,
-Rob


>
> Paolo
>
> > ---
> >  accel/tcg/cpu-exec.c | 34 ++++++++++++++++++++++++++--------
> >  1 file changed, 26 insertions(+), 8 deletions(-)
> >
> > diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> > index 80d0e649b2..cde27ee0bf 100644
> > --- a/accel/tcg/cpu-exec.c
> > +++ b/accel/tcg/cpu-exec.c
> > @@ -517,9 +517,13 @@ static inline bool cpu_handle_exception(CPUState *cpu, int *ret)
> >  #else
> >          if (replay_exception()) {
> >              CPUClass *cc = CPU_GET_CLASS(cpu);
> > -            qemu_mutex_lock_iothread();
> > +            if (cc->bql_interrupt) {
> > +                qemu_mutex_lock_iothread();
> > +            }
> >              cc->do_interrupt(cpu);
> > -            qemu_mutex_unlock_iothread();
> > +            if (cc->bql_interrupt) {
> > +                qemu_mutex_unlock_iothread();
> > +            }
> >              cpu->exception_index = -1;
> >
> >              if (unlikely(cpu->singlestep_enabled)) {
> > @@ -558,7 +562,7 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
> >      if (unlikely(cpu_interrupt_request(cpu))) {
> >          int interrupt_request;
> >
> > -        qemu_mutex_lock_iothread();
> > +        cpu_mutex_lock(cpu);
> >          interrupt_request = cpu_interrupt_request(cpu);
> >          if (unlikely(cpu->singlestep_enabled & SSTEP_NOIRQ)) {
> >              /* Mask out external interrupts for this step. */
> > @@ -567,7 +571,7 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
> >          if (interrupt_request & CPU_INTERRUPT_DEBUG) {
> >              cpu_reset_interrupt(cpu, CPU_INTERRUPT_DEBUG);
> >              cpu->exception_index = EXCP_DEBUG;
> > -            qemu_mutex_unlock_iothread();
> > +            cpu_mutex_unlock(cpu);
> >              return true;
> >          }
> >          if (replay_mode == REPLAY_MODE_PLAY && !replay_has_interrupt()) {
> > @@ -577,13 +581,15 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
> >              cpu_reset_interrupt(cpu, CPU_INTERRUPT_HALT);
> >              cpu_halted_set(cpu, 1);
> >              cpu->exception_index = EXCP_HLT;
> > -            qemu_mutex_unlock_iothread();
> > +            cpu_mutex_unlock(cpu);
> >              return true;
> >          }
> >  #if defined(TARGET_I386)
> >          else if (interrupt_request & CPU_INTERRUPT_INIT) {
> >              X86CPU *x86_cpu = X86_CPU(cpu);
> >              CPUArchState *env = &x86_cpu->env;
> > +            cpu_mutex_unlock(cpu);
> > +            qemu_mutex_lock_iothread();
> >              replay_interrupt();
> >              cpu_svm_check_intercept_param(env, SVM_EXIT_INIT, 0, 0);
> >              do_cpu_init(x86_cpu);
> > @@ -595,7 +601,7 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
> >          else if (interrupt_request & CPU_INTERRUPT_RESET) {
> >              replay_interrupt();
> >              cpu_reset(cpu);
> > -            qemu_mutex_unlock_iothread();
> > +            cpu_mutex_unlock(cpu);
> >              return true;
> >          }
> >  #endif
> > @@ -604,7 +610,15 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
> >             True when it is, and we should restart on a new TB,
> >             and via longjmp via cpu_loop_exit.  */
> >          else {
> > +            cpu_mutex_unlock(cpu);
> > +            if (cc->bql_interrupt) {
> > +                qemu_mutex_lock_iothread();
> > +            }
> >              if (cc->cpu_exec_interrupt(cpu, interrupt_request)) {
> > +                if (cc->bql_interrupt) {
> > +                    qemu_mutex_unlock_iothread();
> > +                }
> > +                cpu_mutex_lock(cpu);
> >                  replay_interrupt();
> >                  /*
> >                   * After processing the interrupt, ensure an EXCP_DEBUG is
> > @@ -614,6 +628,11 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
> >                  cpu->exception_index =
> >                      (cpu->singlestep_enabled ? EXCP_DEBUG : -1);
> >                  *last_tb = NULL;
> > +            } else {
> > +                if (cc->bql_interrupt) {
> > +                    qemu_mutex_unlock_iothread();
> > +                }
> > +                cpu_mutex_lock(cpu);
> >              }
> >              /* The target hook may have updated the 'cpu->interrupt_request';
> >               * reload the 'interrupt_request' value */
> > @@ -627,7 +646,7 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
> >          }
> >
> >          /* If we exit via cpu_loop_exit/longjmp it is reset in cpu_exec */
> > -        qemu_mutex_unlock_iothread();
> > +        cpu_mutex_unlock(cpu);
> >      }
> >
> >      /* Finally, check if we need to exit to the main loop.  */
> > @@ -691,7 +710,6 @@ static inline void cpu_loop_exec_tb(CPUState *cpu, TranslationBlock *tb,
> >      }
> >  #endif
> >  }
> > -
> >  /* main execution loop */
> >
> >  int cpu_exec(CPUState *cpu)
> >
>