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

[Sean Christopherson <seanjc@google.com>]

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.  Which I'm fine with, but if we go
that route, then I'd prefer not to special case non-serializing callers.

E.g. in the KVM code, I find the use of wrmsrns() intuitive, because KVM doesn't
need the WRMSR to be serializing and so can eke out a bit of extra performance by
using wrmsrns() instead of wrmsrq().  But with native_wrmsrq(), it's not clear
why _this_ particular WRMSR in KVM needs to use the "native" version.

There are a pile of other WRMSRs in KVM that are in hot paths, especially with
the mediated PMU support.  If we're going to make the default version non-serializing,
then I'd prefer to get that via wrmsrq(), i.e. reap the benefits for all of KVM,
not just one arbitrary path.

> For the safe/unsafe variants make __wrmsrq() to be a common base
> function instead of duplicating the ALTERNATIVE*() macros. This
> requires to let native_wrmsr() use native_wrmsrq() instead of
> __wrmsrq(). While changing this, convert native_wrmsr() into an inline
> function.
> 
> Replace the only call of wsrmsrns() with the now equivalent call to
> native_wrmsrq() and remove wsrmsrns().

...

> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 3799cbbb4577..e29a2ac24669 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1473,7 +1473,7 @@ static void vmx_write_guest_host_msr(struct vcpu_vmx *vmx, u32 msr, u64 data,
>  {
>  	preempt_disable();
>  	if (vmx->vt.guest_state_loaded)
> -		wrmsrns(msr, data);
> +		native_wrmsrq(msr, data);
>  	preempt_enable();
>  	*cache = data;
>  }
> -- 
> 2.53.0
>