Re: [RFC][PATCH 6/8] module: Add module specific symbol namespace support

[Sean Christopherson <seanjc@google.com>]

On Mon, Nov 11, 2024, Christoph Hellwig wrote:
> On Mon, Nov 11, 2024 at 11:54:36AM +0100, Peter Zijlstra wrote:
> > Designate the "MODULE_${modname}" symbol namespace to mean: 'only
> > export to the named module'.
> > 
> > Notably, explicit imports of anything in the "MODULE_" space is
> > forbidden. Modules implicitly get the "MODULE_${modname}" namespace
> > added.
> 
> Btw, I finally remember why I wanted a separate macro for this:
> so that we can also add the config symbol as an argument and not
> export the symbol if the module isn't configured or built in.

That could get ugly, especially in generic code, as multiple KVM architectures
use multiple modules, e.g. x86 generates kvm.ko, and then vendor specific modules
kvm-amd.ko and kvm-intel.ko; and PPC generates kvm.ko, and kvm-hv.ko and kvm-pr.ko.
PPC in particular is annoying because it generates kvm.ko for KVM_BOOK3S_32=m or
KVM_BOOK3S_64=m.

The other quirk is that, on x86 at least, kvm.ko is now built if and only if at
least one of KVM_AMD=m or KVM_INTEL=m, which triggers KVM_X86=m.  I.e. kvm.ko isn't
built if there are no vendor modules, even if KVM=m.

I'd also like to use this infrastructure to restrict KVM's own exports, e.g. so
that KVM exports its symbols for kvm-{amd,intel,hv,pr}.ko only as needed.

So rather than having EXPORT_SYMBOL_GPL_FOR() deal with KVM's messes, would it
instead make sense to have KVM provide EXPORT_SYMBOL_GPL_FOR_KVM()?  Then KVM can
reuse the painful extrapolation of Kconfigs to module names for its own exports.
And IMO, that'd make the code that does the exports much more readable, too.

E.g. for x86, something like:

#if IS_MODULE(CONFIG_KVM_AMD) && IS_MODULE(CONFIG_KVM_INTEL)
#define KVM_VENDOR_MODULES kvm-amd,kvm-intel
#elif IS_MODULE(CONFIG_KVM_AMD)
#define KVM_VENDOR_MODULES kvm-amd
#elif IS_MODULE(CONFIG_KVM_INTEL)
#define KVM_VENDOR_MODULES kvm-intel
#else
#undef KVM_VENDOR_MODULES
#endif

#ifdef KVM_VENDOR_MODULES
static_assert(IS_MODULE(CONFIG_KVM_X86));

#define EXPORT_SYMBOL_GPL_FOR_KVM_INTERNAL(symbol) \
	EXPORT_SYMBOL_GPL_FOR(symbol, __stringify(KVM_VENDOR_MODULES))
#define EXPORT_SYMBOL_GPL_FOR_KVM(symbol) \
	EXPORT_SYMBOL_GPL_FOR(symbol, "kvm," __stringify(KVM_VENDOR_MODULES))
#else
EXPORT_SYMBOL_GPL_FOR_KVM_INTERNAL(symbol)
EXPORT_SYMBOL_GPL_FOR_KVM(symbol)
#endif