linux-kernel - Re: [PATCH v4 1/1] x86/cpu: Make sure flag_is_changeable

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Message-ID: <B7F5B222-2982-43D5-87A5-7510EDCCB4DF@zytor.com>
Date: Fri, 08 Nov 2024 16:13:05 +0100
From: "H. Peter Anvin" <hpa@...or.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        Justin Stitt <justinstitt@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH v4 1/1] x86/cpu: Make sure flag_is_changeable_p() is always being used

On November 8, 2024 3:11:46 PM GMT+01:00, Andy Shevchenko <andriy.shevchenko@...ux.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, although it currently
>only runs in 32-bit cases.
>
>Suggested-by: Dave Hansen <dave.hansen@...el.com>
>Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
>---
>
>v4: fixed the type of parameter (hpa)
>v3: rewritten as suggested (Dave)
>v2: marked both 32- and 64-bit cases
>
> arch/x86/kernel/cpu/common.c | 33 +++++++++++++++------------------
> 1 file changed, 15 insertions(+), 18 deletions(-)
>
>diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>index a5f221ea5688..49682e35f9bd 100644
>--- a/arch/x86/kernel/cpu/common.c
>+++ b/arch/x86/kernel/cpu/common.c
>@@ -276,22 +276,14 @@ 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 int flag_is_changeable_p(unsigned long flag)
> {
> 	u32 f1, f2;
> 
>+	if (!IS_ENABLED(CONFIG_X86_32))
>+		return 1;
>+
> 	/*
> 	 * Cyrix and IDT cpus allow disabling of CPUID
> 	 * so the code below may return different results
>@@ -316,6 +308,17 @@ static inline int flag_is_changeable_p(u32 flag)
> 	return ((f1^f2) & flag) != 0;
> }
> 
>+#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)
> {
>@@ -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)

It's good that you are changing the return type, but we need to be consistent and change f1, f2 to match. At the same time, I suggest changing the return type to bool.