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Re: Ping: [PATCH] x86: replace a few do_div() uses
- To: Jan Beulich <jbeulich@xxxxxxxx>, Andrew Cooper <amc96@xxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Fri, 18 Feb 2022 13:22:58 +0000
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- Cc: Wei Liu <wl@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
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- Thread-topic: Ping: [PATCH] x86: replace a few do_div() uses
On 18/02/2022 08:39, Jan Beulich wrote:
> On 12.01.2022 10:28, Jan Beulich wrote:
>> 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".
> I think I did address both points (the earlier one was actually more a
> remark imo anyway, not a request to change anything right in this patch),
> so may I please ask for an ack (or a response clarifying what I'm not
> understanding in what you have said)?
No - you're right. My mistake.
Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
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