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Message-ID: <e462f223-7986-4e91-89cb-6274b97c3246@intel.com>
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, ®s[CPUID_EAX], ®s[CPUID_EBX],
>> + ®s[CPUID_ECX], ®s[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.
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