vfio: Drop vfio_file_iommu_group() stub to fudge around a KVM wart

Drop the vfio_file_iommu_group() stub and instead unconditionally declare
the function to fudge around a KVM wart where KVM tries to do symbol_get()
on vfio_file_iommu_group() (and other VFIO symbols) even if CONFIG_VFIO=n.

Ensuring the symbol is always declared fixes a PPC build error when
modules are also disabled, in which case symbol_get() simply points at the
address of the symbol (with some attributes shenanigans).  Because KVM
does symbol_get() instead of directly depending on VFIO, the lack of a
fully defined symbol is not problematic (ugly, but "fine").

   arch/powerpc/kvm/../../../virt/kvm/vfio.c:89:7:
   error: attribute declaration must precede definition [-Werror,-Wignored-attributes]
           fn = symbol_get(vfio_file_iommu_group);
                ^
   include/linux/module.h:805:60: note: expanded from macro 'symbol_get'
   #define symbol_get(x) ({ extern typeof(x) x __attribute__((weak,visibility("hidden"))); &(x); })
                                                              ^
   include/linux/vfio.h:294:35: note: previous definition is here
   static inline struct iommu_group *vfio_file_iommu_group(struct file *file)
                                     ^
   arch/powerpc/kvm/../../../virt/kvm/vfio.c:89:7:
   error: attribute declaration must precede definition [-Werror,-Wignored-attributes]
           fn = symbol_get(vfio_file_iommu_group);
                ^
   include/linux/module.h:805:65: note: expanded from macro 'symbol_get'
   #define symbol_get(x) ({ extern typeof(x) x __attribute__((weak,visibility("hidden"))); &(x); })
                                                                   ^
   include/linux/vfio.h:294:35: note: previous definition is here
   static inline struct iommu_group *vfio_file_iommu_group(struct file *file)
                                     ^
   2 errors generated.

Although KVM is firmly in the wrong (there is zero reason for KVM to build
virt/kvm/vfio.c when VFIO is disabled), fudge around the error in VFIO as
the stub is unnecessary and doesn't serve its intended purpose (KVM is the
only external user of vfio_file_iommu_group()), and there is an in-flight
series to clean up the entire KVM<->VFIO interaction, i.e. fixing this in
KVM would result in more churn in the long run, and the stub needs to go
away regardless.

Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]
Link: https://lore.kernel.org/all/[email protected]
Link: https://lore.kernel.org/all/[email protected]
Cc: Nick Desaulniers <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Tested-by: Michael Ellerman <[email protected]>
Fixes: c1cce6d ("vfio: Compile vfio_group infrastructure optionally")
Signed-off-by: Sean Christopherson <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alex Williamson <[email protected]>

Author: sean-jc

include/linux/vfio.h

@@ -289,16 +289,12 @@ void vfio_combine_iova_ranges(struct rb_root_cached *root, u32 cur_nodes,
 /*
  * External user API
  */
-#if IS_ENABLED(CONFIG_VFIO_GROUP)
 struct iommu_group *vfio_file_iommu_group(struct file *file);
+
+#if IS_ENABLED(CONFIG_VFIO_GROUP)
 bool vfio_file_is_group(struct file *file);
 bool vfio_file_has_dev(struct file *file, struct vfio_device *device);
 #else
-static inline struct iommu_group *vfio_file_iommu_group(struct file *file)
-{
-	return NULL;
-}
-
 static inline bool vfio_file_is_group(struct file *file)
 {
 	return false;