Re: [PATCH v3 09/16] x86/msr: Use the alternatives mechanism for WRMSR

[Jürgen Groß]

On 19.02.26 00:36, H. Peter Anvin wrote:
> On February 18, 2026 1:37:42 PM PST, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> > On 2/18/26 13:00, Sean Christopherson wrote:
> > > On Wed, Feb 18, 2026, Juergen Gross wrote:
> > > > When available use one of the non-serializing WRMSR variants (WRMSRNS
> > > > with or without an immediate operand specifying the MSR register) in
> > > > __wrmsrq().
> > > Silently using a non-serializing version (or not) seems dangerous (not for KVM,
> > > but for the kernel at-large), unless the rule is going to be that MSR writes need
> > > to be treated as non-serializing by default.
> >
> > Yeah, there's no way we can do this in general. It'll work for 99% of
> > the MSRs on 99% of the systems for a long time. Then the one new system
> > with WRMSRNS is going to have one hell of a heisenbug that'll take years
> > off some poor schmuck's life.
> >
> > We should really encourage *new* code to use wrmsrns() when it can at
> > least for annotation that it doesn't need serialization. But I don't
> > think we should do anything to old, working code.
>
> Correct. We need to do this on a user by user basis.

Then I'd prefer to introduce a new wrmsr_sync() function for the serializing
variant and to switch all current users which are not known to tolerate the
non-serializing form to it. The main advantage of that approach would be to
be able to use the immediate form where possible automatically.


Juergen

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