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

[Jan Beulich <jbeulich@xxxxxxxx>]

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().)

> @@ -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.

Jan