Re: [PATCH RFC 0/6] x86/msr: Rename MSR access functions

Peter Zijlstra
2026-04-20T13:10:30Z

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On Mon, Apr 20, 2026 at 03:01:31PM +0200, Jürgen Groß wrote:
> On 20.04.26 14:33, Peter Zijlstra wrote:

> > That is, we have the following instructions: RDMSR, WRMSR, WRMSRNS
> > And we should have the exact same functions:
> >
> > val = rdmsr(msr);
> > wrmsr(msr, val);
> > wrmsrns(msr, val);
>
> People tend to copy similar code, maybe using older kernels as the source.
>
> So even if wrmsrns() would be fine (and, resulting from that, better), they
> will more likely end up using wrmsr() instead.
>
> Using new function names implying the exact semantics (serializing vs.
> non-serializing) will make it more likely the correct one is being used.

You cannot fix stupid. If you want friction, the label thing will
ensure 'old' code doesn't compile and will need fixing.

Also, if wrmsrns() really is faster, the performance folks will finger
'incorrect' wrmsr() usage sooner or later.

> > The only interesting question is what to do with the 'safe' aspect. The
> > instruction takes a fault, we do the extable, but rdmsr() above already
> > has a return value, so that can't be used.
> >
> > One option is to, like uaccess and the proposed overflow, is to use
> > labels like:
> >
> > val = rdmsr(msr, label);
> >
> > And then, even though the wrmsr*() functions have the return available,
> > do we want to be consistent and do:
> >
> > wrmsr(msr, val, label);
> > wrmsrns(msr, val, label);
> >
> > rather than be inconsistent and have them have a boolean return for
> > success.
> >
> > What am I missing?
>
> I like the idea to use a label, but this would result in the need to use
> macros instead of functions. So this is trading one aspect against another.
> I'm not sure which is the better one here.
>
> An alternative might be to switch rdmsr() to the interface used by rdmsr_safe(),
> i.e. let all the accessors return a bool for success/failure and use a pointer
> for the MSR value in rdmsr().

Yes, either way around works. Perhaps that is 'better' because mostly we
don't care about the faults since we've checked the 'feature' earlier.

Its just inconvenient to have return in argument crud, but whatever ;-)

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