Re: [PATCH v7 07/10] x86/vmscape: Use static_call() for predictor flush

[Pawan Gupta]

On Thu, Mar 19, 2026 at 09:58:02PM +0100, Peter Zijlstra wrote:
> On Thu, Mar 19, 2026 at 08:41:54AM -0700, Pawan Gupta wrote:
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index 68e2df3e3bf5..b75eda114503 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -144,6 +144,17 @@ EXPORT_SYMBOL_GPL(cpu_buf_idle_clear);
> >   */
> >  DEFINE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
> >  
> > +/*
> > + * Controls CPU Fill buffer clear before VMenter. This is a subset of
> > + * X86_FEATURE_CLEAR_CPU_BUF, and should only be enabled when KVM-only
> > + * mitigation is required.
> > + */
> > +DEFINE_STATIC_KEY_FALSE(cpu_buf_vm_clear);
> > +EXPORT_SYMBOL_GPL(cpu_buf_vm_clear);
> > +
> > +DEFINE_STATIC_CALL_NULL(vmscape_predictor_flush, write_ibpb);
> > +EXPORT_STATIC_CALL_GPL(vmscape_predictor_flush);
> 
> Does that want to be:
> 
> EXPORT_STATIC_CALL_TRAMP_GPL(vmscape_predictor_flush);
> 
> The distinction being that if you only export the trampoline, modules
> can do the static_call() thing, but cannot do static_call_update().

Right, modules shouldn't be updating this static_call().

One caveat of not exporting the static key is that KVM uses the key to
determine whether the mitigation is deployed or not:

  vcpu_enter_guest()
  {
      ...

     /*
      * Mark this CPU as needing a branch predictor flush before running
      * userspace. Must be done before enabling preemption to ensure it gets
      * set for the CPU that actually ran the guest, and not the CPU that it
      * may migrate to.
      */
     if (static_call_query(vmscape_predictor_flush))
                   this_cpu_write(x86_predictor_flush_exit_to_user, true);

With _TRAMP, KVM complains:

 ERROR: modpost: "__SCK__vmscape_predictor_flush" [arch/x86/kvm/kvm.ko] undefined!

Probably one option is to somehow make sure that the key can be set to
__ro_after_init? I don't see a use case for modifying the static_call() after
boot.