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

[Nicola Vetrini <nicola.vetrini@bugseng.com>]

On 2025-03-26 10:54, Penny, Zheng wrote:
> [Public]
> 
> Hi,
> 
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@suse.com>
>> Sent: Monday, March 24, 2025 11:48 PM
>> To: Penny, Zheng <penny.zheng@amd.com>
>> Cc: Huang, Ray <Ray.Huang@amd.com>; Andrew Cooper
>> <andrew.cooper3@citrix.com>; Roger Pau Monné <roger.pau@citrix.com>; 
>> xen-
>> devel@lists.xenproject.org; Nicola Vetrini 
>> <nicola.vetrini@bugseng.com>
>> Subject: Re: [PATCH v3 07/15] xen/cpufreq: fix core frequency 
>> calculation for AMD
>> Family 1Ah CPUs
>> 
>> 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.

>> Jan

-- 
Nicola Vetrini, B.Sc.
Software Engineer
BUGSENG (https://bugseng.com)
LinkedIn: https://www.linkedin.com/in/nicola-vetrini-a42471253