Re: [PATCH] x86: replace a few do_div() uses

[Jan Beulich <jbeulich@suse.com>]

On 12.01.2022 10:22, Andrew Cooper wrote:
> On 12/01/2022 09:00, Jan Beulich wrote:
>> When the macro's "return value" is not used, the macro use can be
>> replaced by a simply division, avoiding some obfuscation.
>>
>> According to my observations, no change to generated code.
>>
>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> 
> I like this change in principle, but see below.
> 
> do_div() needs to be deleted, because it's far too easy screw up.  At a
> bare minimum, it should be replaced with a static inline that takes it's
> first parameter by pointer, because then at least every callsite reads
> correctly in terms of the C language.

That ought to be a 2nd step, requiring agreement with Arm folks (and
adjustments to their code).

>> --- a/xen/arch/x86/time.c
>> +++ b/xen/arch/x86/time.c
>> @@ -610,8 +610,7 @@ static uint64_t xen_timer_cpu_frequency(
>>      struct vcpu_time_info *info = &this_cpu(vcpu_info)->time;
>>      uint64_t freq;
>>  
>> -    freq = 1000000000ULL << 32;
>> -    do_div(freq, info->tsc_to_system_mul);
>> +    freq = (1000000000ULL << 32) / info->tsc_to_system_mul;
>>      if ( info->tsc_shift < 0 )
>>          freq <<= -info->tsc_shift;
> 
> do_div()'s output is consumed here.  I don't think this hunk is safe to
> convert.

If by "output" you mean its "return value", then it clearly isn't
consumed. And I continue to think that I did express correctly the
effect do_div() did have on "freq".

Jan