Re: [PATCH v5 1/1] x86/cpu: Make sure flag_is_changeable_p() is always being used

["H. Peter Anvin" <hpa@zytor.com>]

On November 8, 2024 4:30:10 PM GMT+01:00, Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote:
>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 6863f5643dd7 ("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>
>---
>
>v5: fixed type of the local vars (hpa), made it return bool (hpa)
>v4: fixed the type of parameter (hpa)
>v3: rewritten as suggested (Dave)
>v2: marked both 32- and 64-bit cases
>
> arch/x86/include/asm/cpuid.h |  8 +++++---
> arch/x86/kernel/cpu/common.c | 39 +++++++++++++++++-------------------
> 2 files changed, 23 insertions(+), 24 deletions(-)
>
>diff --git a/arch/x86/include/asm/cpuid.h b/arch/x86/include/asm/cpuid.h
>index ca4243318aad..239b9ba5c398 100644
>--- a/arch/x86/include/asm/cpuid.h
>+++ b/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,
>diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>index a5f221ea5688..f361336416ae 100644
>--- a/arch/x86/kernel/cpu/common.c
>+++ b/arch/x86/kernel/cpu/common.c
>@@ -276,21 +276,13 @@ 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
>@@ -313,11 +305,22 @@ 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);
> }
>@@ -349,10 +352,6 @@ 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)
> {
> }
>@@ -1088,7 +1087,6 @@ 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;
> 
> 	/*
>@@ -1109,7 +1107,6 @@ static void identify_cpu_without_cpuid(struct cpuinfo_x86 *c)
> 				break;
> 			}
> 		}
>-#endif
> }
> 
> #define NO_SPECULATION		BIT(0)

Looks good to me:

Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>

But another question: why the hell does clang complain about an unused static inline function?!