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Re: [PATCH] x86: replace a few do_div() uses
- To: Andrew Cooper <amc96@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 12 Jan 2022 10:28:59 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 12 Jan 2022 09:45:30 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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@xxxxxxxx>
>
> 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
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