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

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Message-ID: <CAFULd4bs7meRq_iTqq1nSzpkSzEQqLkM8S=x4=zqtX6Fo9qQPQ@mail.gmail.com>
Date: Thu, 18 Jul 2024 10:52:17 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: "H. Peter Anvin" <hpa@...or.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 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.