Re: [PATCH 1/5] xen/domain: Remove function pointers from domain pause helpers

[Andrew Cooper <amc96@srcf.net>]

On 12/11/2021 09:57, Jan Beulich wrote:
> On 11.11.2021 18:57, Andrew Cooper wrote:
>> --- a/xen/common/domain.c
>> +++ b/xen/common/domain.c
>> @@ -1234,15 +1234,18 @@ int vcpu_unpause_by_systemcontroller(struct vcpu *v)
>>       return 0;
>>   }
>>   
>> -static void do_domain_pause(struct domain *d,
>> -                            void (*sleep_fn)(struct vcpu *v))
>> +static void _domain_pause(struct domain *d, bool sync /* or nosync */)
>>   {
>>       struct vcpu *v;
>>   
>>       atomic_inc(&d->pause_count);
>>   
>> -    for_each_vcpu( d, v )
>> -        sleep_fn(v);
>> +    if ( sync )
>> +        for_each_vcpu ( d, v )
>> +            vcpu_sleep_sync(v);
>> +    else
>> +        for_each_vcpu ( d, v )
>> +            vcpu_sleep_nosync(v);
> Is this really better (for whichever reason) than
>
>      for_each_vcpu ( d, v )
>      {
>          if ( sync )
>              vcpu_sleep_sync(v);
>          else
>              vcpu_sleep_nosync(v);
>      }
>
> ?

Yes.  For cases where it can't be optimised out via constant 
propagation, it removes a conditional branch from the middle of a loop.  
I forget what the name for the compiler pass which does this is, but it 
makes a big difference given the way that L0 instruction caches and 
loop-stream-detectors/etc are build.

~Andrew