linux-kernel - Re: [PATCH] x86/boot: Use __ASM_SIZE() to reduce ifdeffery in cpuflags.c

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Message-ID: <77174D9A-79DE-44A7-85E0-63B0BFE343C2@zytor.com>
Date: Thu, 18 Jul 2024 01:55:56 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Uros Bizjak <ubizjak@...il.com>
CC: x86@...nel.org, linux-kernel@...r.kernel.org,
        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: Use __ASM_SIZE() to reduce ifdeffery in cpuflags.c

On July 18, 2024 1:52:17 AM PDT, Uros Bizjak <ubizjak@...il.com> wrote:
>On Thu, Jul 18, 2024 at 8:36 AM H. Peter Anvin <hpa@...or.com> wrote:
>>
>> On July 17, 2024 11:32:18 PM PDT, Uros Bizjak <ubizjak@...il.com> wrote:
>> >Use __ASM_SIZE() macro to add correct insn suffix to pushf/popf.
>> >
>> >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 | 10 +++-------
>> > 1 file changed, 3 insertions(+), 7 deletions(-)
>> >
>> >diff --git a/arch/x86/boot/cpuflags.c b/arch/x86/boot/cpuflags.c
>> >index d75237ba7ce9..aacabe431fd5 100644
>> >--- a/arch/x86/boot/cpuflags.c
>> >+++ b/arch/x86/boot/cpuflags.c
>> >@@ -2,6 +2,7 @@
>> > #include <linux/types.h>
>> > #include "bitops.h"
>> >
>> >+#include <asm/asm.h>
>> > #include <asm/processor-flags.h>
>> > #include <asm/required-features.h>
>> > #include <asm/msr-index.h>
>> >@@ -36,13 +37,8 @@ static int has_fpu(void)
>> >  * compressed/ directory where it may be 64-bit code, and thus needs
>> >  * to be 'pushfq' or 'popfq' in that case.
>> >  */
>> >-#ifdef __x86_64__
>> >-#define PUSHF "pushfq"
>> >-#define POPF "popfq"
>> >-#else
>> >-#define PUSHF "pushfl"
>> >-#define POPF "popfl"
>> >-#endif
>> >+#define PUSHF __ASM_SIZE(pushf)
>> >+#define POPF __ASM_SIZE(popf)
>> >
>> > int has_eflag(unsigned long mask)
>> > {
>>
>> Just use pushf/popf. gas hasn't needed that suffix for a long time as far as I know.
>
>Unfortunately, clang does not do the right thing when pushf/popf
>without suffix are used.
>
>arch/x86/boot/cpuflags.c compiles to:
>
>00000000 <has_eflag>:
>   0:    9c                       pushf
>   1:    9c                       pushf
>   2:    66 5a                    pop    %edx
>   4:    66 89 d1                 mov    %edx,%ecx
>   7:    66 31 c1                 xor    %eax,%ecx
>   a:    66 51                    push   %ecx
>   c:    9d                       popf
>   d:    9c                       pushf
>   e:    66 59                    pop    %ecx
>  10:    9d                       popf
>  11:    66 31 ca                 xor    %ecx,%edx
>  14:    66 31 c9                 xor    %ecx,%ecx
>  17:    66 85 c2                 test   %eax,%edx
>  1a:    0f 95 c1                 setne  %cl
>  1d:    66 89 c8                 mov    %ecx,%eax
>  20:    66 c3                    retl
>
>instead of:
>
>00000000 <has_eflag>:
>   0:    66 9c                    pushfl
>   2:    66 9c                    pushfl
>   4:    66 5a                    pop    %edx
>   6:    66 89 d1                 mov    %edx,%ecx
>   9:    66 31 c1                 xor    %eax,%ecx
>   c:    66 51                    push   %ecx
>   e:    66 9d                    popfl
>  10:    66 9c                    pushfl
>  12:    66 59                    pop    %ecx
>  14:    66 9d                    popfl
>  16:    66 31 ca                 xor    %ecx,%edx
>  19:    66 31 c9                 xor    %ecx,%ecx
>  1c:    66 85 c2                 test   %eax,%edx
>  1f:    0f 95 c1                 setne  %cl
>  22:    66 89 c8                 mov    %ecx,%eax
>  25:    66 c3                    retl
>
>Please note missing 0x66 operand size override prefixes with pushfl
>and popfl. This is 16bit code, operand prefixes are mandatory to push
>32-bit EFLAGS register (ID flag lives in bit 21).
>
>So, the original patch is the way to go.
>
>Uros.
>

You do know that has_eflag can be completely elided on x86-64, or you can use %z with one of the register operands.

One more reason why clang really needs to shape up.