Re: [Qemu-devel] [PATCH v7 18/27] cputlb: introduce tlb_flush_*_all_cpus

[Alex Bennée]

Richard Henderson <address@hidden> writes:

> On 01/24/2017 12:34 PM, Alex Bennée wrote:
>>
>> Richard Henderson <address@hidden> writes:
>>
>>> On 01/19/2017 09:04 AM, Alex Bennée wrote:
>>>> +/* flush_all_helper: run fn across all cpus
>>>> + *
>>>> + * If the wait flag is set then the src cpu's helper will be queued as
>>>> + * "safe" work and the loop exited creating a synchronisation point
>>>> + * where all queued work will be finished before execution starts
>>>> + * again.
>>>> + */
>>>> +static void flush_all_helper(CPUState *src, bool wait,
>>>> +                             run_on_cpu_func fn, run_on_cpu_data d)
>>>> +{
>>>> +    CPUState *cpu;
>>>> +
>>>> +    if (!wait) {
>>>> +        CPU_FOREACH(cpu) {
>>>> +            if (cpu != src) {
>>>> +                async_run_on_cpu(cpu, fn, d);
>>>> +            } else {
>>>> +                g_assert(qemu_cpu_is_self(src));
>>>> +                fn(src, d);
>>>> +            }
>>>> +        }
>>>> +    } else {
>>>> +        CPU_FOREACH(cpu) {
>>>> +            if (cpu != src) {
>>>> +                async_run_on_cpu(cpu, fn, d);
>>>> +            } else {
>>>> +                async_safe_run_on_cpu(cpu, fn, d);
>>>> +            }
>>>> +
>>>> +        }
>>>> +        cpu_loop_exit(src);
>>>> +    }
>>>> +}
>>>
>>> What's the rationale for not having the target do the exit itself?  Surely 
>>> it
>>> can tell, and simple end the TB after the insn.
>>
>> It's more for the global sync functionality. I wanted to keep all the
>> guts of re-starting the loop with the correct async_safe_work all in one
>> place with a defined API for the guests rather than have them all do it
>> themselves.
>>
>> For the common case of not needing to sync across the cores I agree the
>> guest is perfectly able to end the TB so its safe work completes next.
>> In fact the ARM helper calls do exactly that.
>
> Hmm.  Would it make more sense to have two functions then, for wait and !wait?
> That would allow the wait function be QEMU_NORETURN, which might make it a bit
> more obvious about the interface contract.

Seems fair. I was worried about multiplying out too many variants in the
API but this seems a good reason to.

--
Alex Bennée