linux-kernel - Re: [PATCH 1/4] x86/cpu: Add and use new CPUID region helper

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Date: Tue, 2 Apr 2024 10:13:31 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Cc: "tglx@...utronix.de" <tglx@...utronix.de>,
 "jgross@...e.com" <jgross@...e.com>, "x86@...nel.org" <x86@...nel.org>,
 "bp@...en8.de" <bp@...en8.de>
Subject: Re: [PATCH 1/4] x86/cpu: Add and use new CPUID region helper

On 3/25/24 05:24, Huang, Kai wrote:
> 
> Nit:
> 
>> +
>> +/* Returns true if the leaf exists and @value was populated */
> 
> 						 ^ is ?

It's a subtle difference, but I think it's better as I wrote it.
Returning true happens *after* the value _was_ populated.

>> +static inline bool get_cpuid_region_leaf(u32 leaf, enum cpuid_regs_idx reg,
>> +					 u32 *value)
>> +{
>> +	u16 region = leaf >> 16;
>> +	u32 regs[4];
>> +
>> +	if (cpuid_region_max_leaf(region) < leaf)
>> +		return false;
>> +
>> +	cpuid(leaf, &regs[CPUID_EAX], &regs[CPUID_EBX],
>> +	            &regs[CPUID_ECX], &regs[CPUID_EDX]);
>> +
>> +	*value = regs[reg];
>> +
>> +	return true;
>> +}
> 
> I found despite the get_cpuid_region_leaf() returns true/false, the return value
> is never used in this series.  Instead, this series uses below pattern:
> 
> 	u32 data = 0; 	/* explicit initialization */
> 
> 	get_cpuid_region_leaf(leaf, ..., &data);
> 
> Which kinda implies the 'data' won't be touched if the requested leaf isn't
> supported I suppose?
> 
> Since the return value is never used, should we consider just making this
> function void?

I certainly considered it.

But I do think that get_cpuid_region_leaf() looks a lot more obviously
correct and useful when it explicitly returns what it did, even if the
existing callers don't take advantage of it.

I suspect it generates the same code either way.