linux-kernel - Re: [PATCH] x86/boot: Do not test if AC and ID eflags are changeable on x86

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Message-ID: <DC668A65-3173-4A0C-BF78-1CECF60D300B@zytor.com>
Date: Fri, 07 Mar 2025 03:58:31 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Uros Bizjak <ubizjak@...il.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH] x86/boot: Do not test if AC and ID eflags are changeable on x86_64

On March 7, 2025 1:10:03 AM PST, Uros Bizjak <ubizjak@...il.com> wrote:
>The test for the changeabitily of AC and ID eflags is used to
>distinguish between i386 and i486 processors (AC) and to test
>for cpuid instruction support (ID).
>
>Skip these tests on x86_64 processors as they always supports cpuid.
>
>Also change the return type of has_eflag() to bool.
>
>Signed-off-by: Uros Bizjak <ubizjak@...il.com>
>Cc: Thomas Gleixner <tglx@...utronix.de>
>Cc: Ingo Molnar <mingo@...nel.org>
>Cc: Borislav Petkov <bp@...en8.de>
>Cc: Dave Hansen <dave.hansen@...ux.intel.com>
>Cc: "H. Peter Anvin" <hpa@...or.com>
>---
> arch/x86/boot/cpuflags.c | 26 +++++++++-----------------
> arch/x86/boot/cpuflags.h |  6 +++++-
> 2 files changed, 14 insertions(+), 18 deletions(-)
>
>diff --git a/arch/x86/boot/cpuflags.c b/arch/x86/boot/cpuflags.c
>index d75237ba7ce9..2150a016176f 100644
>--- a/arch/x86/boot/cpuflags.c
>+++ b/arch/x86/boot/cpuflags.c
>@@ -29,40 +29,32 @@ static int has_fpu(void)
> 	return fsw == 0 && (fcw & 0x103f) == 0x003f;
> }
> 
>+#ifdef CONFIG_X86_32
> /*
>  * For building the 16-bit code we want to explicitly specify 32-bit
>  * push/pop operations, rather than just saying 'pushf' or 'popf' and
>- * letting the compiler choose. But this is also included from the
>- * compressed/ directory where it may be 64-bit code, and thus needs
>- * to be 'pushfq' or 'popfq' in that case.
>+ * letting the compiler choose.
>  */
>-#ifdef __x86_64__
>-#define PUSHF "pushfq"
>-#define POPF "popfq"
>-#else
>-#define PUSHF "pushfl"
>-#define POPF "popfl"
>-#endif
>-
>-int has_eflag(unsigned long mask)
>+bool has_eflag(unsigned long mask)
> {
> 	unsigned long f0, f1;
> 
>-	asm volatile(PUSHF "	\n\t"
>-		     PUSHF "	\n\t"
>+	asm volatile("pushfl	\n\t"
>+		     "pushfl	\n\t"
> 		     "pop %0	\n\t"
> 		     "mov %0,%1	\n\t"
> 		     "xor %2,%1	\n\t"
> 		     "push %1	\n\t"
>-		     POPF "	\n\t"
>-		     PUSHF "	\n\t"
>+		     "popfl	\n\t"
>+		     "pushfl	\n\t"
> 		     "pop %1	\n\t"
>-		     POPF
>+		     "popfl"
> 		     : "=&r" (f0), "=&r" (f1)
> 		     : "ri" (mask));
> 
> 	return !!((f0^f1) & mask);
> }
>+#endif
> 
> void cpuid_count(u32 id, u32 count, u32 *a, u32 *b, u32 *c, u32 *d)
> {
>diff --git a/arch/x86/boot/cpuflags.h b/arch/x86/boot/cpuflags.h
>index fdcc2aa4c3c4..a398d9204ad0 100644
>--- a/arch/x86/boot/cpuflags.h
>+++ b/arch/x86/boot/cpuflags.h
>@@ -15,7 +15,11 @@ struct cpu_features {
> extern struct cpu_features cpu;
> extern u32 cpu_vendor[3];
> 
>-int has_eflag(unsigned long mask);
>+#ifdef CONFIG_X86_32
>+bool has_eflag(unsigned long mask);
>+#else
>+static inline bool has_eflag(unsigned long mask) { return true; }
>+#endif
> void get_cpuflags(void);
> void cpuid_count(u32 id, u32 count, u32 *a, u32 *b, u32 *c, u32 *d);
> bool has_cpuflag(int flag);

PUSF et al → pushf

The -l and -q suffixes have been optional for a long time.