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Message-ID: <CAFULd4b+co40kxWbjuY4C_uGB9MukSNKdB1t7umqtpWVNEzxKQ@mail.gmail.com>
Date: Wed, 21 Jan 2026 11:03:48 +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 02/14] x86/realmode: make %gs == 0 an invariant
On Tue, Jan 20, 2026 at 8:54 PM H. Peter Anvin <hpa@...or.com> wrote:
>
> When accessing data that is not "near", either only one segment is
> used or one segment is always zero. Leave %gs == 0 at all times
> throughout the C code; this reduces the number of segment loads
> needed.
>
> Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.com>
Reviewed-by: Uros Bizjak <ubizjak@...il.com>
> ---
> arch/x86/boot/a20.c | 11 +++++------
> arch/x86/boot/header.S | 3 +++
> arch/x86/boot/regs.c | 3 ++-
> arch/x86/boot/video-bios.c | 5 ++---
> arch/x86/boot/video-mode.c | 5 ++---
> arch/x86/boot/video.c | 7 +++----
> arch/x86/realmode/rm/wakeup_asm.S | 10 ++++++----
> 7 files changed, 23 insertions(+), 21 deletions(-)
>
> diff --git a/arch/x86/boot/a20.c b/arch/x86/boot/a20.c
> index bda042933a05..3ab6cd8eaa31 100644
> --- a/arch/x86/boot/a20.c
> +++ b/arch/x86/boot/a20.c
> @@ -56,20 +56,19 @@ static int a20_test(int loops)
> int ok = 0;
> int saved, ctr;
>
> - set_fs(0x0000);
> - set_gs(0xffff);
> + set_fs(0xffff);
>
> - saved = ctr = rdfs32(A20_TEST_ADDR);
> + saved = ctr = rdgs32(A20_TEST_ADDR);
>
> while (loops--) {
> - wrfs32(++ctr, A20_TEST_ADDR);
> + wrgs32(++ctr, A20_TEST_ADDR);
> io_delay(); /* Serialize and make delay constant */
> - ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr;
> + ok = rdfs32(A20_TEST_ADDR+0x10) ^ ctr;
> if (ok)
> break;
> }
>
> - wrfs32(saved, A20_TEST_ADDR);
> + wrgs32(saved, A20_TEST_ADDR);
> return ok;
> }
>
> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index 9bea5a1e2c52..bda20395658f 100644
> --- a/arch/x86/boot/header.S
> +++ b/arch/x86/boot/header.S
> @@ -596,6 +596,9 @@ start_of_setup:
> shrw $2, %cx
> rep stosl
>
> +# The C code uses %gs == 0 as invariant
> + movw %ax, %gs
> +
> # Jump to C code (should not return)
> calll main
>
> diff --git a/arch/x86/boot/regs.c b/arch/x86/boot/regs.c
> index 55de6b3092b8..54d6dfd129c5 100644
> --- a/arch/x86/boot/regs.c
> +++ b/arch/x86/boot/regs.c
> @@ -22,6 +22,7 @@ void initregs(struct biosregs *reg)
> reg->eflags |= X86_EFLAGS_CF;
> reg->ds = ds();
> reg->es = ds();
> + /* The input values of %cs and %ss are ignored by intcall() */
> reg->fs = fs();
> - reg->gs = gs();
> + /* %gs == 0 */
> }
> diff --git a/arch/x86/boot/video-bios.c b/arch/x86/boot/video-bios.c
> index 6eb8c06bc287..e8be64424a40 100644
> --- a/arch/x86/boot/video-bios.c
> +++ b/arch/x86/boot/video-bios.c
> @@ -73,7 +73,6 @@ static int bios_probe(void)
> if (adapter != ADAPTER_EGA && adapter != ADAPTER_VGA)
> return 0;
>
> - set_fs(0);
> crtc = vga_crtc();
>
> video_bios.modes = GET_HEAP(struct mode_info, 0);
> @@ -105,8 +104,8 @@ static int bios_probe(void)
> mi = GET_HEAP(struct mode_info, 1);
> mi->mode = VIDEO_FIRST_BIOS+mode;
> mi->depth = 0; /* text */
> - mi->x = rdfs16(0x44a);
> - mi->y = rdfs8(0x484)+1;
> + mi->x = rdgs16(0x44a);
> + mi->y = rdgs8(0x484)+1;
> nmodes++;
> }
>
> diff --git a/arch/x86/boot/video-mode.c b/arch/x86/boot/video-mode.c
> index 9ada55dc1ab7..e5b9bc96bd42 100644
> --- a/arch/x86/boot/video-mode.c
> +++ b/arch/x86/boot/video-mode.c
> @@ -119,9 +119,8 @@ static void vga_recalc_vertical(void)
> u16 crtc;
> u8 pt, ov;
>
> - set_fs(0);
> - font_size = rdfs8(0x485); /* BIOS: font size (pixels) */
> - rows = force_y ? force_y : rdfs8(0x484)+1; /* Text rows */
> + font_size = rdgs8(0x485); /* BIOS: font size (pixels) */
> + rows = force_y ? force_y : rdgs8(0x484)+1; /* Text rows */
>
> rows *= font_size; /* Visible scan lines */
> rows--; /* ... minus one */
> diff --git a/arch/x86/boot/video.c b/arch/x86/boot/video.c
> index 0641c8c46aee..09b810faa5c0 100644
> --- a/arch/x86/boot/video.c
> +++ b/arch/x86/boot/video.c
> @@ -79,12 +79,11 @@ static void store_mode_params(void)
> video_segment = 0xb800;
> }
>
> - set_fs(0);
> - font_size = rdfs16(0x485); /* Font size, BIOS area */
> + font_size = rdgs16(0x485); /* Font size, BIOS area */
> boot_params.screen_info.orig_video_points = font_size;
>
> - x = rdfs16(0x44a);
> - y = (adapter == ADAPTER_CGA) ? 25 : rdfs8(0x484)+1;
> + x = rdgs16(0x44a);
> + y = (adapter == ADAPTER_CGA) ? 25 : rdgs8(0x484)+1;
>
> if (force_x)
> x = force_x;
> diff --git a/arch/x86/realmode/rm/wakeup_asm.S b/arch/x86/realmode/rm/wakeup_asm.S
> index 02d0ba16ae33..a8a8580158d7 100644
> --- a/arch/x86/realmode/rm/wakeup_asm.S
> +++ b/arch/x86/realmode/rm/wakeup_asm.S
> @@ -70,15 +70,17 @@ SYM_CODE_START(wakeup_start)
> movl $rm_stack_end, %esp
> movw %ax, %ds
> movw %ax, %es
> - movw %ax, %fs
> - movw %ax, %gs
>
> - lidtl .Lwakeup_idt
> + xorl %eax, %eax
> + movw %ax, %fs
> + movw %ax, %gs /* The real mode code requires %gs == 0 */
>
> /* Clear the EFLAGS */
> - pushl $0
> + pushl %eax
> popfl
>
> + lidtl .Lwakeup_idt
> +
> /* Check header signature... */
> movl signature, %eax
> cmpl $WAKEUP_HEADER_SIGNATURE, %eax
> --
> 2.52.0
>
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