x86/cpu: Make sure flag_is_changeable_p() is always being used

When flag_is_changeable_p() is unused, it prevents kernel builds
with clang, `make W=1` and CONFIG_WERROR=y:

arch/x86/kernel/cpu/common.c:351:19: error: unused function 'flag_is_changeable_p' [-Werror,-Wunused-function]
  351 | static inline int flag_is_changeable_p(u32 flag)
      |                   ^~~~~~~~~~~~~~~~~~~~

Fix this by moving core around to make sure flag_is_changeable_p() is
always being used.

See also commit 6863f56 ("kbuild: allow Clang to find unused static
inline functions for W=1 build").

While at it, fix the argument type to be unsigned long along with
the local variables, although it currently only runs in 32-bit cases.
Besides that, makes it return boolean instead of int. This induces
the change of the returning type of have_cpuid_p() to be boolean
as well.

Suggested-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Link: https://lore.kernel.org/all/20241108153105.1578186-1-andriy.shevchenko%40linux.intel.com

Author: andy-shev

arch/x86/include/asm/cpuid.h

@@ -6,6 +6,8 @@
 #ifndef _ASM_X86_CPUID_H
 #define _ASM_X86_CPUID_H
 
+#include <linux/types.h>
+
 #include <asm/string.h>
 
 struct cpuid_regs {
@@ -20,11 +22,11 @@ enum cpuid_regs_idx {
 };
 
 #ifdef CONFIG_X86_32
-extern int have_cpuid_p(void);
+bool have_cpuid_p(void);
 #else
-static inline int have_cpuid_p(void)
+static inline bool have_cpuid_p(void)
 {
-	return 1;
+	return true;
 }
 #endif
 static inline void native_cpuid(unsigned int *eax, unsigned int *ebx,

arch/x86/kernel/cpu/common.c

@@ -275,21 +275,13 @@ static int __init x86_noinvpcid_setup(char *s)
 }
 early_param("noinvpcid", x86_noinvpcid_setup);
 
-#ifdef CONFIG_X86_32
-static int cachesize_override = -1;
-static int disable_x86_serial_nr = 1;
-
-static int __init cachesize_setup(char *str)
-{
-	get_option(&str, &cachesize_override);
-	return 1;
-}
-__setup("cachesize=", cachesize_setup);
-
 /* Standard macro to see if a specific flag is changeable */
-static inline int flag_is_changeable_p(u32 flag)
+static inline bool flag_is_changeable_p(unsigned long flag)
 {
-	u32 f1, f2;
+	unsigned long f1, f2;
+
+	if (!IS_ENABLED(CONFIG_X86_32))
+		return true;
 
 	/*
 	 * Cyrix and IDT cpus allow disabling of CPUID
@@ -312,11 +304,22 @@ static inline int flag_is_changeable_p(u32 flag)
 		      : "=&r" (f1), "=&r" (f2)
 		      : "ir" (flag));
 
-	return ((f1^f2) & flag) != 0;
+	return (f1 ^ f2) & flag;
 }
 
+#ifdef CONFIG_X86_32
+static int cachesize_override = -1;
+static int disable_x86_serial_nr = 1;
+
+static int __init cachesize_setup(char *str)
+{
+	get_option(&str, &cachesize_override);
+	return 1;
+}
+__setup("cachesize=", cachesize_setup);
+
 /* Probe for the CPUID instruction */
-int have_cpuid_p(void)
+bool have_cpuid_p(void)
 {
 	return flag_is_changeable_p(X86_EFLAGS_ID);
 }
@@ -348,10 +351,6 @@ static int __init x86_serial_nr_setup(char *s)
 }
 __setup("serialnumber", x86_serial_nr_setup);
 #else
-static inline int flag_is_changeable_p(u32 flag)
-{
-	return 1;
-}
 static inline void squash_the_stupid_serial_number(struct cpuinfo_x86 *c)
 {
 }
@@ -1087,7 +1086,6 @@ void get_cpu_address_sizes(struct cpuinfo_x86 *c)
 
 static void identify_cpu_without_cpuid(struct cpuinfo_x86 *c)
 {
-#ifdef CONFIG_X86_32
 	int i;
 
 	/*
@@ -1108,7 +1106,6 @@ static void identify_cpu_without_cpuid(struct cpuinfo_x86 *c)
 				break;
 			}
 		}
-#endif
 }
 
 #define NO_SPECULATION		BIT(0)