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

[Pawan Gupta]

On Tue, Jul 28, 2026 at 11:52:41AM -0700, Sean Christopherson wrote:
> On Wed, Jun 24, 2026, 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@xxxxxxxxxxxxx>
> > > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> > > ---
> > >  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.
> 
> *sigh*
> 
> So the patches build, but they aren't fully functional.  If kvm.ko is built as a
> module, trying to load a vendor module will fail due to the trampoline => key table
> only tracking built-in symbols.
> 
> Given that all of this is KVM, and that it's far easier to export the static calls
> if and only if there are vendor modules, and only for those vendor modules, I'm
> going to drop the aggressive cleanup and abandon converting KVM's exports to only
> export trampolines, at least until someone else works up the motivation to add
> support for modules.

Thanks for the update. Keeping this patch in the series.