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

[H. Peter Anvin]

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.