[RFC PATCH 1/2] percpu: do not put btf_type_tag on per-cpu variable declarations

[Vineet Gupta <vineet.gupta@linux.dev>]

__percpu carries BTF_TYPE_TAG(percpu), which records that a pointer
points to per-cpu memory. __PCPU_ATTRS() applies __percpu to the per-cpu
variable itself, so DECLARE_PER_CPU() and DEFINE_PER_CPU() put the tag in
declaration specifier position on the variable's own type.

arch/x86/include/asm/processor.h has

	DECLARE_PER_CPU_READ_MOSTLY(struct cpuinfo_x86, cpu_info);

which an x86 defconfig preprocesses to

	extern __seg_gs __attribute__((btf_type_tag("percpu")))
	__attribute__((section(".data..percpu" "..read_mostly")))
	__typeof__(struct cpuinfo_x86) cpu_info;

and GCC warns:

	processor.h:232:36: warning: ignoring attributes applied to
	'struct cpuinfo_x86' after definition [-Wattributes]

which -Werror turns into a build failure.

The message is confusing: this is a declaration, not a definition. But
the compiler is right to object to a type tag on the aggregate, and the
issue is fundamental: every variant of an aggregate shares one DIE, so
there is nowhere to put it. clang drops it silently and GCC warns.

The tag is meant to record that a pointer points to per-cpu memory, and
cpu_info is not a pointer.

And this gets worse: for a pointer-typed per-cpu variable GCC attaches
the tag to the pointee, so

	static DEFINE_PER_CPU(struct bpf_hrtimer *, hrtimer_running);

comes out as PTR -> TYPE_TAG 'percpu' -> struct bpf_hrtimer. The truth
is the other way round: hrtimer_running lives in per-cpu memory and
points at ordinary memory, while that record says it is an ordinary
pointer to per-cpu memory. clang emits no tag there at all.

Add __percpu_var, which skips the btf tag (__percpu under sparse) and
use it in __PCPU_ATTRS(). The cpu_info declaration above becomes

	extern __seg_gs
	__attribute__((section(".data..percpu" "..read_mostly")))
	__typeof__(struct cpuinfo_x86) cpu_info;

The pointer spelling, T __percpu *p, is unchanged and keeps its tag.

Before this patch the tag also qualified the variable's type, so &var
had type "T __percpu *", and a pointer does have a DIE of its own:
pahole emitted PTR -> TYPE_TAG 'percpu' -> T for each one.
this_cpu_ptr(&var) is that shape, and that is where the records came
from.

Those are the records an x86 defconfig built with clang loses: 280 BTF
types, 140 TYPE_TAG and the 140 pointers carrying them, with .BTF
shrinking from 6831660 to 6828300 bytes. They are the derived pointers
-- clang never recorded the declarations themselves. None of them were
reachable: a record points at a type only if something is declared with
it, and nothing is declared with these. The 794 places that do declare a
"T __percpu *" keep theirs -- the tags reached by a STRUCT, UNION,
ARRAY, FUNC_PROTO or another PTR number 117, 6, 3, 39 and 5 before and
after. .text is byte-identical.

Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
---
 include/linux/compiler_types.h | 8 ++++++++
 include/linux/percpu-defs.h    | 2 +-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index c5921f139007..0a18b3f10675 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -50,6 +50,7 @@
 # define __user		__attribute__((noderef, address_space(__user)))
 # define __iomem	__attribute__((noderef, address_space(__iomem)))
 # define __percpu	__attribute__((noderef, address_space(__percpu)))
+# define __percpu_var	__percpu
 # define __rcu		__attribute__((noderef, address_space(__rcu)))
 static inline void __chk_user_ptr(const volatile void __user *ptr) { }
 static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
@@ -68,7 +69,14 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
 #  define __user	BTF_TYPE_TAG(user)
 # endif
 # define __iomem
+/*
+ * btf_type_tag records "points to per-cpu memory", so it belongs on the
+ * pointer spelling only. Declaring the per-cpu variable itself puts the tag
+ * in declaration specifier position, where clang drops it and GCC attaches it
+ * to the wrong type, so __percpu_var leaves it out.
+ */
 # define __percpu	__percpu_qual BTF_TYPE_TAG(percpu)
+# define __percpu_var	__percpu_qual
 # define __rcu		BTF_TYPE_TAG(rcu)
 
 # define __chk_user_ptr(x)	(void)0
diff --git a/include/linux/percpu-defs.h b/include/linux/percpu-defs.h
index dbe3267a0a13..f53fc430aaad 100644
--- a/include/linux/percpu-defs.h
+++ b/include/linux/percpu-defs.h
@@ -45,7 +45,7 @@
  * that section.
  */
 #define __PCPU_ATTRS(sec)						\
-	__percpu __attribute__((section(PER_CPU_BASE_SECTION sec)))	\
+	__percpu_var __attribute__((section(PER_CPU_BASE_SECTION sec)))	\
 	PER_CPU_ATTRIBUTES
 
 #define __PCPU_DUMMY_ATTRS						\
-- 
2.53.0-Meta