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

[Peter Zijlstra]

On Thu, Mar 19, 2026 at 02:34:21PM -0700, Pawan Gupta wrote:
> 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!

Ah, tricky. Yeah, this would need to be solved differenlty. Perhaps wrap
this in a helper and export that?

Or use the below little thing and change it to
EXPORT_STATIC_CALL_FOR_MODULES(foo, "kvm"); or whatnot.

> 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.

So we have __ro_after_init for static_branch, but we'd not done
it for static_call yet. It shouldn't be terribly difficult, just hasn't
been done. Not sure this is the moment to do so.


---
diff --git a/include/linux/static_call.h b/include/linux/static_call.h
index 78a77a4ae0ea..b610afd1ed55 100644
--- a/include/linux/static_call.h
+++ b/include/linux/static_call.h
@@ -216,6 +216,9 @@ extern long __static_call_return0(void);
 #define EXPORT_STATIC_CALL_GPL(name)					\
 	EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name));			\
 	EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
+#define EXPORT_STATIC_CALL_FOR_MODULES(name, mods)			\
+	EXPORT_SYMBOL_FOR_MODULES(STATIC_CALL_KEY(name), mods);		\
+	EXPORT_SYMBOL_FOR_MODULES(STATIC_CALL_TRAMP(name), mods)
 
 /* Leave the key unexported, so modules can't change static call targets: */
 #define EXPORT_STATIC_CALL_TRAMP(name)					\
@@ -276,6 +279,9 @@ extern long __static_call_return0(void);
 #define EXPORT_STATIC_CALL_GPL(name)					\
 	EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name));			\
 	EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
+#define EXPORT_STATIC_CALL_FOR_MODULES(name, mods)			\
+	EXPORT_SYMBOL_FOR_MODULES(STATIC_CALL_KEY(name), mods);		\
+	EXPORT_SYMBOL_FOR_MODULES(STATIC_CALL_TRAMP(name), mods)
 
 /* Leave the key unexported, so modules can't change static call targets: */
 #define EXPORT_STATIC_CALL_TRAMP(name)					\
@@ -346,6 +352,8 @@ static inline int static_call_text_reserved(void *start, void *end)
 
 #define EXPORT_STATIC_CALL(name)	EXPORT_SYMBOL(STATIC_CALL_KEY(name))
 #define EXPORT_STATIC_CALL_GPL(name)	EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name))
+#define EXPORT_STATIC_CALL_FOR_MODULES(name, mods)			\
+	EXPORT_SYMBOL_FOR_MODULES(STATIC_CALL_KEY(name), mods)
 
 #endif /* CONFIG_HAVE_STATIC_CALL */