Re: [PATCH v3 07/15] xen/cpufreq: fix core frequency calculation for AMD Family 1Ah CPUs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.03.2025 11:14, Nicola Vetrini wrote:
> On 2025-03-26 10:54, Penny, Zheng wrote:
>>> -----Original Message-----
>>> From: Jan Beulich <jbeulich@xxxxxxxx>
>>> Sent: Monday, March 24, 2025 11:48 PM
>>>
>>> On 06.03.2025 09:39, Penny Zheng wrote:
>>>> This commit fixes core frequency calculation for AMD Family 1Ah CPUs,
>>>> due to a change in the PStateDef MSR layout in AMD Family 1Ah+.
>>>> In AMD Family 1Ah+, Core current operating frequency in MHz is
>>>> calculated as
>>>> follows:
>>>
>>> Why 1Ah+? In the code you correctly limit to just 1Ah.
>>>
>>>> --- a/xen/arch/x86/cpu/amd.c
>>>> +++ b/xen/arch/x86/cpu/amd.c
>>>> @@ -572,12 +572,24 @@ static void amd_get_topology(struct cpuinfo_x86 *c)
>>>>                                                            :
>>>> c->cpu_core_id);  }
>>>>
>>>> +static uint64_t amd_parse_freq(const struct cpuinfo_x86 *c, uint64_t
>>>> +value) {
>>>> +   ASSERT(c->x86 <= 0x1A);
>>>> +
>>>> +   if (c->x86 < 0x17)
>>>> +           return (((value & 0x3f) + 0x10) * 100) >> ((value >> 6) & 7);
>>>> +   else if (c->x86 <= 0x19)
>>>> +           return ((value & 0xff) * 25 * 8) / ((value >> 8) & 0x3f);
>>>> +   else
>>>> +           return (value & 0xfff) * 5;
>>>> +}
>>>
>>> Could I talk you into omitting the unnecessary "else" in cases like 
>>> this one?
>>> (This may also make sense to express as switch().)
>>>
>>
>> Sorry, bad habit... will change it to switch
>>
>>>> @@ -658,19 +670,20 @@ void amd_log_freq(const struct cpuinfo_x86 *c)
>>>>     if (!(lo >> 63))
>>>>             return;
>>>>
>>>> -#define FREQ(v) (c->x86 < 0x17 ? ((((v) & 0x3f) + 0x10) * 100) >> (((v) 
>>>> >> 6) &
>>> 7) \
>>>> -                                : (((v) & 0xff) * 25 * 8) / (((v) >> 8) & 
>>>> 0x3f))
>>>>     if (idx && idx < h &&
>>>>         !rdmsr_safe(0xC0010064 + idx, val) && (val >> 63) &&
>>>>         !rdmsr_safe(0xC0010064, hi) && (hi >> 63))
>>>>             printk("CPU%u: %lu (%lu ... %lu) MHz\n",
>>>> -                  smp_processor_id(), FREQ(val), FREQ(lo), FREQ(hi));
>>>> +                  smp_processor_id(),
>>>> +                  amd_parse_freq(c, val),
>>>> +                  amd_parse_freq(c, lo), amd_parse_freq(c, hi));
>>>
>>> I fear Misra won't like multiple function calls to evaluate the 
>>> parameters to pass to
>>> another function. Iirc smp_process_id() has special exception, so 
>>> that's okay here.
>>> This may be possible to alleviate by marking the new helper pure or 
>>> even const
>>> (see gcc doc as to caveats with passing pointers to const functions). 
>>> Cc-ing Nicola
>>> for possible clarification or correction.
>>>
>>
>> Maybe we shall declare the function __pure. Having checked the gcc doc,
>> ``
>> a function that has pointer arguments must not be declared const
>> ``
>> Otherwise we store the "c->x86" value to avoid using the pointer
> 
> Either way could work. ECLAIR will automatically pick up 
> __attribute__((pure)) or __attribute__((const)) from the declaration. 
> Maybe it could be const, as from a cursory look I don't think the gcc 
> restriction on pointer arguments applies, as the pointee is not modified 
> between successive calls, but I might be mistaken.

Indeed this matches my reading of it. Yet things are somewhat delicate here,
so I like to always leave room for being proven wrong.

Jan