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

[Sean Christopherson]

On Fri, Feb 20, 2026, Xin Li wrote:
> 
> > On Feb 18, 2026, at 10:44 PM, Jürgen Groß <jgross@xxxxxxxx> wrote:
> > 
> > On 18.02.26 22:37, Dave Hansen 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.
> > 
> > I _really_ thought this was discussed upfront by Xin before he sent out his
> > first version of the series.
> 
> I actually reached out to the Intel architects about this before I started
> coding. Turns out, if the CPU supports WRMSRNS, you can use it across the
> board.  The hardware is smart enough to perform a serialized write whenever
> a non-serialized one isn't proper, so there’s no risk.

How can hardware possibly know what's "proper"?  E.g. I don't see how hardware
can reason about safety if there's a software sequence that is subtly relying on
the serialization of WRMSR to provide some form of ordering.

And if that's the _architectural_ behavior, then what's the point of WRMSRNS?
If it's not architectural, then I don't see how the kernel can rely on it.