Re: [PATCH v12 07/12] static_call: Define EXPORT_STATIC_CALL_FOR_MODULES()

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

On Wed, Jun 24, 2026 at 05:59:19AM -0700, Sean Christopherson wrote:
> On Tue, Jun 23, 2026, Pawan Gupta wrote:
> > There is EXPORT_STATIC_CALL_TRAMP() that hides the static key from all
> > modules. But there is no equivalent of EXPORT_SYMBOL_FOR_MODULES() to
> > restrict symbol visibility to only certain modules.
> > 
> > Add EXPORT_STATIC_CALL_FOR_MODULES(name, mods) that wraps both the key and
> > the trampoline with EXPORT_SYMBOL_FOR_MODULES(), allowing only a limited
> > set of modules to see and update the static key.
> > 
> > The immediate user is KVM, in the following commit.
> > 
> > checkpatch reported below warnings with this change that I believe don't
> > apply in this case:
> > 
> >   include/linux/static_call.h:219: WARNING: Non-declarative macros with multiple statements should be enclosed in a do - while loop
> >   include/linux/static_call.h:220: WARNING: EXPORT_SYMBOL(foo); should immediately follow its function/variable
> > 
> > Suggested-by: Peter Zijlstra <peterz@infradead.org>
> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> > ---
> >  include/linux/static_call.h | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > 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 */
> 
> Drat, I forgot about this.  Exporting static call trampolines for KVM came up in
> another conversation[*].  I had already put together patches to effectively default
> to exporting only the trampoline, and also to deduplicate this code so that the
> CONFIG_HAVE_STATIC_CALL_INLINE=y / CONFIG_HAVE_STATIC_CALL=y / CONFIG_HAVE_STATIC_CALL=n
> implementations don't need to copy+paste the same lines of code.
> 
> The attached patches touch a lot more code, and will conflict mightily with KVM
> changes I want to land in 7.3 (more use of a static_call in KVM).  But if we get
> them applied (to tip tree) shortly after 7.2-rc1 and provide a topic branch/tag,
> then there shouldn't be too much juggling needed?
> 
> If we want to go with the more aggressive cleanup, I'll formally post the patches.
> 
> [*] https://lore.kernel.org/all/ahhoDGUz39KSGZ6o@google.com

Thanks for the context.

Earlier making the key ro-after-init came up as an option in a thread with
Peter. Does it look like a good option to you?

diff --git a/include/linux/static_call.h b/include/linux/static_call.h
index b610afd1ed55..ea56da8fb446 100644
--- a/include/linux/static_call.h
+++ b/include/linux/static_call.h
@@ -200,6 +200,14 @@ extern long __static_call_return0(void);
 	};								\
 	ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
 
+#define DEFINE_STATIC_CALL_NULL_RO_AFTER_INIT(name, _func)		\
+	DECLARE_STATIC_CALL(name, _func);				\
+	struct static_call_key STATIC_CALL_KEY(name) __ro_after_init = {\
+		.func = _func,						\
+		.type = 1,						\
+	};								\
+	ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
+
 #define DEFINE_STATIC_CALL_RET0(name, _func)				\
 	DECLARE_STATIC_CALL(name, _func);				\
 	struct static_call_key STATIC_CALL_KEY(name) = {		\