Re: [PATCH 4/5] x86/cpuid: Standardize on u32 in <asm/cpuid/api.h>

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 18/03/2025 6:48 pm, H. Peter Anvin wrote:
> One more thing is that we ought to be able to make cpuid a const function, allowing the compiler to elide multiple calls. (Slight warning for feature-enabling MSRs changing CPUID), but that would require changing the API to returning a structure, since a pure or const structure can't return values by reference.

It's not only the feature-enabling MSRs.  It's also OSXSAVE/OSPKE/etc in
CR4, and on Intel CPUs, the CPUID instruction still has a side effect
for microcode patch revision MSR.

There are a few too many side effects to call it const/pure.

That said, when experimenting with the same in Xen, there was nothing
interesting the compiler could do with const/pure because of how the
existing logic is laid out.  Removing volatile and the memory clobber
however did allow the compiler to make slightly better code.

~Andrew