Re: [PATCH 06/20] x86/msr: Standardize on 'u32' MSR indices in

Xin Li
2025-04-10T03:18:46Z

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