[PATCH v1 02/14] x86/realmode: make %gs == 0 an invariant

["H. Peter Anvin" <hpa@zytor.com>]

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@zytor.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