percpu: Sanitize __percpu_qual include hell

Slapping __percpu_qual into the next available header is sloppy at best.

It's required by __percpu which is defined in compiler_types.h and that is
meant to be included without requiring a boatload of other headers so that
a struct or function declaration can contain a __percpu qualifier w/o
further prerequisites.

This implicit dependency on linux/percpu.h makes that impossible and causes
a major problem when trying to separate headers.

Create asm/percpu_types.h and move it there. Include that from
compiler_types.h and the whole recursion problem goes away.

Fix up UM so it uses the generic header and includes it in the UM_HOST
build, which pulls in compiler_types.h. The USER_CFLAGS fix was suggested
by Richard.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260602090535.254874125@kernel.org

arch/um/Makefile

@@ -71,7 +71,8 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
 		-D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \
 		-idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__ \
 		-include $(srctree)/include/linux/compiler-version.h \
-		-include $(srctree)/include/linux/kconfig.h
+		-include $(srctree)/include/linux/kconfig.h \
+		-idirafter $(ARCH_DIR)/include/generated
 
 #This will adjust *FLAGS accordingly to the platform.
 include $(srctree)/$(ARCH_DIR)/Makefile-os-Linux

arch/um/include/asm/Kbuild

@@ -16,6 +16,7 @@ generic-y += module.h
 generic-y += module.lds.h
 generic-y += parport.h
 generic-y += percpu.h
+generic-y += percpu_types.h
 generic-y += preempt.h
 generic-y += runtime-const.h
 generic-y += softirq_stack.h

arch/x86/include/asm/percpu.h

@@ -40,12 +40,10 @@
 #endif
 
 #define __percpu_prefix
-#define __percpu_seg_override	CONCATENATE(__seg_, __percpu_seg)
 
 #else /* !CONFIG_CC_HAS_NAMED_AS: */
 
 #define __percpu_prefix		__force_percpu_prefix
-#define __percpu_seg_override
 
 #endif /* CONFIG_CC_HAS_NAMED_AS */
 
@@ -82,7 +80,6 @@
 
 #define __force_percpu_prefix
 #define __percpu_prefix
-#define __percpu_seg_override
 
 #define PER_CPU_VAR(var)	(var)__percpu_rel
 
@@ -92,8 +89,6 @@
 # define __my_cpu_type(var)	typeof(var)
 # define __my_cpu_ptr(ptr)	(ptr)
 # define __my_cpu_var(var)	(var)
-
-# define __percpu_qual		__percpu_seg_override
 #else
 # define __my_cpu_type(var)	typeof(var) __percpu_seg_override
 # define __my_cpu_ptr(ptr)	(__my_cpu_type(*(ptr))*)(__force uintptr_t)(ptr)

arch/x86/include/asm/percpu_types.h

@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_PERCPU_TYPES_H
+#define _ASM_X86_PERCPU_TYPES_H
+
+#if defined(CONFIG_SMP) && defined(CONFIG_CC_HAS_NAMED_AS)
+#define __percpu_seg_override	CONCATENATE(__seg_, __percpu_seg)
+#else /* !CONFIG_CC_HAS_NAMED_AS: */
+#define __percpu_seg_override
+#endif
+
+#if defined(CONFIG_USE_X86_SEG_SUPPORT) && defined(USE_TYPEOF_UNQUAL)
+#define __percpu_qual		__percpu_seg_override
+#endif
+
+#include <asm-generic/percpu_types.h>
+
+#endif

include/asm-generic/Kbuild

@@ -44,6 +44,7 @@ mandatory-y += module.lds.h
 mandatory-y += msi.h
 mandatory-y += pci.h
 mandatory-y += percpu.h
+mandatory-y += percpu_types.h
 mandatory-y += pgalloc.h
 mandatory-y += preempt.h
 mandatory-y += rqspinlock.h

include/asm-generic/percpu_types.h

@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_PERCPU_TYPES_H_
+#define _ASM_GENERIC_PERCPU_TYPES_H_
+
+#ifndef __ASSEMBLER__
+/*
+ * __percpu_qual is the qualifier for the percpu named address space.
+ *
+ * Most architectures use generic named address space for percpu variables but
+ * some architectures define percpu variables in different named address space.
+ * E.g. on x86, 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
+
+#endif /* __ASSEMBLER__ */
+#endif /* _ASM_GENERIC_PERCPU_TYPES_H_ */

include/linux/compiler_types.h

@@ -634,6 +634,9 @@ struct ftrace_likely_data {
 #else
 #define __unqual_scalar_typeof(x) __typeof_unqual__(x)
 #endif
+
+#include <asm/percpu_types.h>
+
 #endif /* !__ASSEMBLY__ */
 
 /*

include/linux/percpu.h

@@ -3,13 +3,14 @@
 #define __LINUX_PERCPU_H
 
 #include <linux/alloc_tag.h>
-#include <linux/mmdebug.h>
-#include <linux/preempt.h>
-#include <linux/smp.h>
-#include <linux/pfn.h>
-#include <linux/init.h>
 #include <linux/cleanup.h>
+#include <linux/compiler_types.h>
+#include <linux/init.h>
+#include <linux/mmdebug.h>
+#include <linux/pfn.h>
+#include <linux/preempt.h>
 #include <linux/sched.h>
+#include <linux/smp.h>
 
 #include <asm/percpu.h>