linux-kernel - Re: [PATCH 06/20] x86/msr: Standardize on 'u32' MSR indices in <asm/msr.h>

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Message-ID: <63ab3ea9-c55d-48d2-8344-fb4314baf240@zytor.com>
Date: Wed, 9 Apr 2025 20:18:12 -0700
From: Xin Li <xin@...or.com>
To: "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
        linux-kernel@...r.kernel.org
Cc: Juergen Gross <jgross@...e.com>, Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 06/20] x86/msr: Standardize on 'u32' MSR indices in
 <asm/msr.h>

On 4/9/2025 6:37 PM, H. Peter Anvin wrote:
> On 4/9/25 14:55, Xin Li wrote:
>>
>> It looks to me that we don't use the "const" qualifier in the code a
>> lot.  However since the MSR index is usually not expected to change
>> inside the MSR API implementations, would it be nicer to add the "const"
>> qualifier?
>>
>> The same to the MSR value of MSR write APIs.
>>
> 
> "const" on an automatic variable (including function arguments) is 
> usually not very meaningful, unless it is manifest as a memory object 
> (see below.)
> 
> Personally I tend to use "const" anyway in more complex functions to 
> make it clear that a variable is not expected to change while in scope 
> (and I also prefer to reduce the scope of a variable as much as 
> possible), but for a simple function like this it is more clutter than 
> anything else.

Good point!

A question NOT related to this patch set, the MSR write API prototype
defined in struct pv_cpu_ops as:
     void (*write_msr)(unsigned int msr, unsigned low, unsigned high);

Will it be better to add "const" to its arguments?  I.e.,
     void (*write_msr)(const u32 msr, const u32 low, const u32 high);

> Now, "const" on a *memory object* (pointer target) is a very different 
> thing and should in general be used where ever writing to an object is 
> not going to happen.
> 
> An automatic variable becomes manifest as a memory object if its address 
> is taken anywhere in its scope (using the & operator or an unindexed 
> array) and the address of that pointer stored. The last part means that 
> the compiler (if it is is smart enough) can take a sequence of 
> operations equivalent to *& and eliminate it.
> 
> Keep in mind that, for C (not necessarily C++):
> 
> 1. in *all* cases "foo[x]" is exactly equivalent to "*(foo + x)"
> 2. *if* "foo" is declared as an array type, then "foo" is exactly 
> equivalent to "&foo[0]".
> 
> "const" in C a little less strict than you would like; the only way in C 
> before C23 to declare a "true" constant is using enum or a #define macro 
> (which of course pollutes the global namespace). In block scope it 
> usually doesn't matter for scalar types and const or static const will 
> work just fine, but it is only in C23 than C imported "constexpr" from 
> C++ (which has had it since C++11.)
Nice to know.
Thanks!
     Xin