Re: [PATCH 3/3] accel/tcg: Make TCGCPUOps::cpu_exec_halt mandatory

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 11/6/24 10:36, Peter Maydell wrote:
> On Tue, 11 Jun 2024 at 09:25, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>>
>> Hi Peter,
>>
>> On 3/6/24 18:09, Peter Maydell wrote:
>>> Now that all targets set TCGCPUOps::cpu_exec_halt, we can make it
>>> mandatory and remove the fallback handling that calls cpu_has_work.
>>>
>>> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
>>> ---
>>>    include/hw/core/tcg-cpu-ops.h | 9 ++++++---
>>>    accel/tcg/cpu-exec.c          | 7 +------
>>>    2 files changed, 7 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/include/hw/core/tcg-cpu-ops.h b/include/hw/core/tcg-cpu-ops.h
>>> index 099de3375e3..34318cf0e60 100644
>>> --- a/include/hw/core/tcg-cpu-ops.h
>>> +++ b/include/hw/core/tcg-cpu-ops.h
>>> @@ -122,10 +122,13 @@ struct TCGCPUOps {
>>>         * to do when the CPU is in the halted state.
>>>         *
>>>         * Return true to indicate that the CPU should now leave halt, false
>>> -     * if it should remain in the halted state.
>>> +     * if it should remain in the halted state. (This should generally
>>> +     * be the same value that cpu_has_work() would return.)
>>>         *
>>> -     * If this method is not provided, the default is to do nothing, and
>>> -     * to leave halt if cpu_has_work() returns true.
>>> +     * This method must be provided. If the target does not need to
>>> +     * do anything special for halt, the same function used for its
>>> +     * CPUClass::has_work method can be used here, as they have the
>>> +     * same function signature.
>>>         */
>>>        bool (*cpu_exec_halt)(CPUState *cpu);
>>>        /**
>>> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
>>> index 6711b58e0b2..8be4d2a1330 100644
>>> --- a/accel/tcg/cpu-exec.c
>>> +++ b/accel/tcg/cpu-exec.c
>>> @@ -682,13 +682,8 @@ 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;
>>> +        bool leave_halt = tcg_ops->cpu_exec_halt(cpu);
>>>
>>> -        if (tcg_ops->cpu_exec_halt) {
>>> -            leave_halt = tcg_ops->cpu_exec_halt(cpu);
>>> -        } else {
>>> -            leave_halt = cpu_has_work(cpu);
>>> -        }
>>>            if (!leave_halt) {
>>>                return true;
>>>            }
>>
>> Could we assert the handler is assigned in tcg_exec_realizefn()?
> 
> Yeah, we could. I thought about an assert that it was set up,
> but couldn't identify a place to do that.
> 
>> If you agree I could squash these 3 lines:
>>
>> -- >8 --
>> --- a/accel/tcg/cpu-exec.c
>> +++ b/accel/tcg/cpu-exec.c
>> @@ -1077,6 +1077,10 @@ bool tcg_exec_realizefn(CPUState *cpu, Error **errp)
>>        static bool tcg_target_initialized;
>>
>>        if (!tcg_target_initialized) {
>> +        /* Check mandatory TCGCPUOps handlers */
>> +        assert(cpu->cc->tcg_ops->initialize);
>> +        assert(cpu->cc->tcg_ops->cpu_exec_halt);
>> +
>>            cpu->cc->tcg_ops->initialize();
> 
> I don't think we need to assert initialize if we're about to call
> it anyway -- the call will crash if it's NULL in an easy to diagnose way.

Pro of assert: obvious error message on stderr.

Con of crash: we need to use a debugger to figure out the NULL deref.

Anyway, series queued without the "assert(initialize)" squashed,

Thanks!

>>            tcg_target_initialized = true;
>>        }
>> ---
>>
>> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> 
> thanks
> -- PMM