linux-kernel - Re: [PATCH v1 12/14] x86/boot: tweak a20.c for better code generation

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Message-ID: <CAFULd4Z+fhbqMmA_OcmuC3qs98iihAyGN3-ktikbpGAt6xWAAA@mail.gmail.com>
Date: Wed, 21 Jan 2026 11:10:34 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Thomas Gleixner <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Petr Mladek <pmladek@...e.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <kees@...nel.org>, 
	"Peter Zijlstra (Intel)" <peterz@...radead.org>, Nathan Chancellor <nathan@...nel.org>, 
	Kiryl Shutsemau <kas@...nel.org>, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev, x86@...nel.org
Subject: Re: [PATCH v1 12/14] x86/boot: tweak a20.c for better code generation

On Tue, Jan 20, 2026 at 8:54 PM H. Peter Anvin <hpa@...or.com> wrote:
>
> Do some minor tweaks to arch/x86/boot/a20.c for better code
> generation, made possible by the __seg_fs/__seg_gs changes.
>
> Move the die() call to a20.c itself; there is no reason to push an
> error code upwards just to die() there.
>
> Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.com>

Acked-by: Uros Bizjak <ubizjak@...il.com>

> ---
>  arch/x86/boot/a20.c  | 24 +++++++++++-------------
>  arch/x86/boot/boot.h |  2 +-
>  arch/x86/boot/pm.c   |  3 +--
>  3 files changed, 13 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/boot/a20.c b/arch/x86/boot/a20.c
> index 52c3fccdcb70..38a1cad8a553 100644
> --- a/arch/x86/boot/a20.c
> +++ b/arch/x86/boot/a20.c
> @@ -53,24 +53,22 @@ static int empty_8042(void)
>
>  static int a20_test(int loops)
>  {
> -       int ok = 0;
>         int saved, ctr;
>
>         set_fs(0xffff);
>
>         saved = ctr = rdgs32(A20_TEST_ADDR);
>
> -       while (loops--) {
> +       do {
>                 wrgs32(++ctr, A20_TEST_ADDR);
>                 io_delay();     /* Serialize and make delay constant */
>                 barrier();      /* Compiler won't know about fs/gs overlap */
> -               ok = rdfs32(A20_TEST_ADDR+0x10) ^ ctr;
> -               if (ok)
> +               if (rdfs32(A20_TEST_ADDR+0x10) != ctr)
>                         break;
> -       }
> +       } while (--loops);
>
>         wrgs32(saved, A20_TEST_ADDR);
> -       return ok;
> +       return loops;
>  }
>
>  /* Quick test to see if A20 is already enabled */
> @@ -125,7 +123,7 @@ static void enable_a20_fast(void)
>
>  #define A20_ENABLE_LOOPS 255   /* Number of times to try */
>
> -int enable_a20(void)
> +void enable_a20(void)
>  {
>         int loops = A20_ENABLE_LOOPS;
>         int kbc_err;
> @@ -134,30 +132,30 @@ int enable_a20(void)
>                /* First, check to see if A20 is already enabled
>                   (legacy free, etc.) */
>                if (a20_test_short())
> -                      return 0;
> +                      return;
>
>                /* Next, try the BIOS (INT 0x15, AX=0x2401) */
>                enable_a20_bios();
>                if (a20_test_short())
> -                      return 0;
> +                      return;
>
>                /* Try enabling A20 through the keyboard controller */
>                kbc_err = empty_8042();
>
>                if (a20_test_short())
> -                      return 0; /* BIOS worked, but with delayed reaction */
> +                      return; /* BIOS worked, but with delayed reaction */
>
>                if (!kbc_err) {
>                        enable_a20_kbc();
>                        if (a20_test_long())
> -                              return 0;
> +                             return;
>                }
>
>                /* Finally, try enabling the "fast A20 gate" */
>                enable_a20_fast();
>                if (a20_test_long())
> -                      return 0;
> +                      return;
>         }
>
> -       return -1;
> +       die("A20 gate not responding, unable to boot...\n");
>  }
> diff --git a/arch/x86/boot/boot.h b/arch/x86/boot/boot.h
> index 4d3549ed7987..584c89d0738b 100644
> --- a/arch/x86/boot/boot.h
> +++ b/arch/x86/boot/boot.h
> @@ -167,7 +167,7 @@ void copy_to_fs(addr_t dst, void *src, size_t len);
>  void *copy_from_fs(void *dst, addr_t src, size_t len);
>
>  /* a20.c */
> -int enable_a20(void);
> +void enable_a20(void);
>
>  /* apm.c */
>  int query_apm_bios(void);
> diff --git a/arch/x86/boot/pm.c b/arch/x86/boot/pm.c
> index 3be89ba4b1b3..e39689ed65ea 100644
> --- a/arch/x86/boot/pm.c
> +++ b/arch/x86/boot/pm.c
> @@ -106,8 +106,7 @@ void go_to_protected_mode(void)
>         realmode_switch_hook();
>
>         /* Enable the A20 gate */
> -       if (enable_a20())
> -               die("A20 gate not responding, unable to boot...\n");
> +       enable_a20();
>
>         /* Reset coprocessor (IGNNE#) */
>         reset_coprocessor();
> --
> 2.52.0
>