percpu: repurpose __percpu tag as a named address space qualifier

The patch introduces __percpu_qual define and repurposes __percpu tag as a
named address space qualifier using the new define.

Arches can now conditionally define __percpu_qual as their named address
space qualifier for percpu variables.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Uros Bizjak <[e-mail]>
Acked-by: Nadav Amit <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Dennis Zhou <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Boqun Feng <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: "David S. Miller" <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: Eric Dumazet <[e-mail]>
Cc: Jakub Kicinski <[e-mail]>
Cc: Kent Overstreet <[e-mail]>
Cc: Paolo Abeni <[e-mail]>
Cc: Waiman Long <[e-mail]>
Cc: Will Deacon <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/asm-generic/percpu.h

···
 #include <linux/threads.h>
 #include <linux/percpu-defs.h>
 
+/*
+ * __percpu_qual is the qualifier for the percpu named address space.
+ *
+ * Most arches use generic named address space for percpu variables but
+ * some arches define percpu variables in different named address space
+ * (on the x86 arch, percpu variable may be declared as being relative
+ * to the %fs or %gs segments using __seg_fs or __seg_gs named address
+ * space qualifier).
+ */
+#ifndef __percpu_qual
+# define __percpu_qual
+#endif
+
 #ifdef CONFIG_SMP
 
 /*

include/linux/compiler_types.h

···
 #  define __user	BTF_TYPE_TAG(user)
 # endif
 # define __iomem
-# define __percpu	BTF_TYPE_TAG(percpu)
+# define __percpu	__percpu_qual BTF_TYPE_TAG(percpu)
 # define __rcu		BTF_TYPE_TAG(rcu)
 
 # define __chk_user_ptr(x)	(void)0