Re: [PATCH 1/2] accel/tcg: Make TCGCPUOps::cpu_exec_halt return bool for whether to halt

["Alex Bennée" <alex.bennee@linaro.org>]

Peter Maydell <peter.maydell@linaro.org> writes:

> The TCGCPUOps::cpu_exec_halt method is called from cpu_handle_halt()
> when the CPU is halted, so that a target CPU emulation can do
> anything target-specific it needs to do.  (At the moment we only use
> this on i386.)
>
> The current specification of the method doesn't allow the target
> specific code to do something different if the CPU is about to come
> out of the halt state, because cpu_handle_halt() only determines this
> after the method has returned.  (If the method called cpu_has_work()
> itself this would introduce a potential race if an interrupt arrived
> between the target's method implementation checking and
> cpu_handle_halt() repeating the check.)
>
> Change the definition of the method so that it returns a bool to
> tell cpu_handle_halt() whether to stay in halt or not.
>
> We will want this for the Arm target, where FEAT_WFxT wants to do
> some work only for the case where the CPU is in halt but about to
> leave it.
>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>  include/hw/core/tcg-cpu-ops.h       | 11 +++++++++--
>  target/i386/tcg/helper-tcg.h        |  2 +-
>  accel/tcg/cpu-exec.c                |  7 +++++--
>  target/i386/tcg/sysemu/seg_helper.c |  3 ++-
>  4 files changed, 17 insertions(+), 6 deletions(-)
>
> diff --git a/include/hw/core/tcg-cpu-ops.h b/include/hw/core/tcg-cpu-ops.h
> index dc1f16a9777..f3ac76e6f6d 100644
> --- a/include/hw/core/tcg-cpu-ops.h
> +++ b/include/hw/core/tcg-cpu-ops.h
> @@ -111,8 +111,15 @@ struct TCGCPUOps {
>      void (*do_interrupt)(CPUState *cpu);
>      /** @cpu_exec_interrupt: Callback for processing interrupts in cpu_exec */
>      bool (*cpu_exec_interrupt)(CPUState *cpu, int interrupt_request);
> -    /** @cpu_exec_halt: Callback for handling halt in cpu_exec */
> -    void (*cpu_exec_halt)(CPUState *cpu);
> +    /**
> +     * @cpu_exec_halt: Callback for handling halt in cpu_exec.
> +     *
> +     * Return true to indicate that the CPU should now leave halt, false
> +     * if it should remain in the halted state.
> +     * If this method is not provided, the default is to leave halt
> +     * if cpu_has_work() returns true.
> +     */
> +    bool (*cpu_exec_halt)(CPUState *cpu);

Would it be too much to rename the method to cpu_exec_leave_halt() to
make it clearer on use the sense of the return value?

>      /**
>       * @tlb_fill: Handle a softmmu tlb miss
>       *
> diff --git a/target/i386/tcg/helper-tcg.h b/target/i386/tcg/helper-tcg.h
> index effc2c1c984..85957943bf3 100644
> --- a/target/i386/tcg/helper-tcg.h
> +++ b/target/i386/tcg/helper-tcg.h
> @@ -39,7 +39,7 @@ QEMU_BUILD_BUG_ON(TCG_PHYS_ADDR_BITS > TARGET_PHYS_ADDR_SPACE_BITS);
>   */
>  void x86_cpu_do_interrupt(CPUState *cpu);
>  #ifndef CONFIG_USER_ONLY
> -void x86_cpu_exec_halt(CPUState *cpu);
> +bool x86_cpu_exec_halt(CPUState *cpu);
>  bool x86_need_replay_interrupt(int interrupt_request);
>  bool x86_cpu_exec_interrupt(CPUState *cpu, int int_req);
>  #endif
> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> index 5c70748060a..550f93b19ce 100644
> --- a/accel/tcg/cpu-exec.c
> +++ b/accel/tcg/cpu-exec.c
> @@ -669,11 +669,14 @@ static inline bool cpu_handle_halt(CPUState *cpu)
>  #ifndef CONFIG_USER_ONLY
>      if (cpu->halted) {
>          const TCGCPUOps *tcg_ops = cpu->cc->tcg_ops;
> +        bool leave_halt;
>  
>          if (tcg_ops->cpu_exec_halt) {
> -            tcg_ops->cpu_exec_halt(cpu);
> +            leave_halt = tcg_ops->cpu_exec_halt(cpu);
> +        } else {
> +            leave_halt = cpu_has_work(cpu);
>          }
> -        if (!cpu_has_work(cpu)) {
> +        if (!leave_halt) {
>              return true;
>          }
>  
> diff --git a/target/i386/tcg/sysemu/seg_helper.c b/target/i386/tcg/sysemu/seg_helper.c
> index 2db8083748e..9ba94deb3aa 100644
> --- a/target/i386/tcg/sysemu/seg_helper.c
> +++ b/target/i386/tcg/sysemu/seg_helper.c
> @@ -128,7 +128,7 @@ void x86_cpu_do_interrupt(CPUState *cs)
>      }
>  }
>  
> -void x86_cpu_exec_halt(CPUState *cpu)
> +bool x86_cpu_exec_halt(CPUState *cpu)
>  {
>      if (cpu->interrupt_request & CPU_INTERRUPT_POLL) {
>          X86CPU *x86_cpu = X86_CPU(cpu);
> @@ -138,6 +138,7 @@ void x86_cpu_exec_halt(CPUState *cpu)
>          cpu_reset_interrupt(cpu, CPU_INTERRUPT_POLL);
>          bql_unlock();
>      }
> +    return cpu_has_work(cpu);

The x86 version is essentially being called for side effects. Do we want
to document this usage in the method?

>  }
>  
>  bool x86_need_replay_interrupt(int interrupt_request)

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro