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

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Message-ID: <0A743606-067F-450C-85A5-55ECE4378E22@zytor.com>
Date: Thu, 18 Jul 2024 13:53:16 -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 2:12:07 AM PDT, Uros Bizjak <ubizjak@...il.com> wrote:
>On Thu, Jul 18, 2024 at 10:56 AM H. Peter Anvin <hpa@...or.com> wrote:
>>
>> 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.
>
>It is 32-bit PUSHFL insn that requires the "L" suffix in 16-bit code.
>This is x86_32 issue and clang was just lucky that the instruction was
>always defined with explicit L suffix. Please note that PUSHF has no
>register operand, so %z can't be used.
>
>> One more reason why clang really needs to shape up.
>
>Indeed.
>
>Thanks,
>Uros.
>

Oh, I see – it is a bug in clang -m16 (.code16gcc or its equivalent.)

However, there you can always do pushfl/popfl since that doesn't apply to 64-bit mode: both AC and ID always exist in 64-bit mode and any future features should be detected by cpuid rather than by frobbing random bits in xFLAGS.

I suggest have_eflag_ac() and have_eflag_id() and just have them return true on 64 bits.