From 8fcabe224198a5c9697fb9a48cc966a50ccedbda Mon Sep 17 00:00:00 2001
From: Damien Zammit via Bug reports for the GNU Hurd <bug-hurd@gnu.org>
Date: Sat, 21 Dec 2024 23:55:17 +0000
Subject: cpuboot: Fix percpu apboot_gdt for early gs

This allocates a constant space for percpu gdts
and copies the first entry to the nth entry on each cpu,
then patches its own copy of the gdt so it can function
independently.
Message-ID: <20241221235456.423860-2-damien@zamaudio.com>
---
 i386/i386/cpuboot.S | 63 ++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 53 insertions(+), 10 deletions(-)

diff --git a/i386/i386/cpuboot.S b/i386/i386/cpuboot.S
index f93e0387..76ed907d 100644
--- a/i386/i386/cpuboot.S
+++ b/i386/i386/cpuboot.S
@@ -31,15 +31,19 @@
 #define BOOT_CS		0x8
 #define BOOT_DS		0x10
 
+#define GDT_DESCR_M32	4
+#define GDT_TABLE_M32	(14*2)
+
 .data
 
 .align 16
 apboot_idt_ptr:
 	.long 0
 .align 16
+apboot_gdt_top:
 	.word 0
 apboot_gdt_descr:
-	.word 14*8-1
+	.word (GDT_TABLE_M32 * 4) - 1
 	.long apboot_gdt - KERNELBASE
 .align 16
 apboot_gdt:
@@ -101,6 +105,9 @@ apboot_percpu_med:
 apboot_percpu_high:
 	.byte 0
 
+/* Empty space for per-cpu gdt descriptor and gdt */
+.space (NCPUS-1) * (GDT_DESCR_M32 + GDT_TABLE_M32) * 4, 0x0
+
 .globl apboot, apbootend, gdt_descr_tmp, apboot_jmp_offset
 .align 16
 .code16
@@ -159,28 +166,62 @@ apboot_jmp_offset:
 	movw	%ax, %gs
 	movw	%ax, %ss
 
-	/* Get CPU number */
+	/* Get CPU number into ebp */
 	movl	$1, %eax
 	cpuid
 	shrl	$24, %ebx
-	movl	%cs:CX(cpu_id_lut, %ebx), %ecx
+	movl	%cs:CX(cpu_id_lut, %ebx), %ebp
+
+	/* Copy first gdt descriptor and gdt to cpu-th area */
+	movl	$(GDT_DESCR_M32 + GDT_TABLE_M32), %ecx
+	movl	$apboot_gdt_top, %esi
+	movl	%esi, %edi
+	movl	$((GDT_DESCR_M32 + GDT_TABLE_M32) * 4), %eax
+	mul	%ebp
+	addl	%eax, %edi
+	rep movsl
 
 	/* Access per_cpu area */
-	movl	%ecx,%eax
+	movl	%ebp, %eax
 	movl	$PC_SIZE,%ebx
 	mul	%ebx
 	addl	$percpu_array, %eax
 
 	/* Record our cpu number */
-	movl	%ecx, (PERCPU_CPU_ID)(%eax)
+	movl	%ebp, (PERCPU_CPU_ID)(%eax)
 
 	/* Set up temporary percpu descriptor */
 	addl	$(-KERNELBASE), %eax
-	movw	%ax, apboot_percpu_low
-	shr	$16, %eax
-	movb	%al, apboot_percpu_med
-	shr	$8, %ax
-	movb	%al, apboot_percpu_high
+	movl	%eax, %ebx
+
+	/* Make eax hold offset to my cpus gdt */
+	movl	$((GDT_DESCR_M32 + GDT_TABLE_M32) * 4), %eax
+	mul	%ebp
+
+	/* Patch only our own copy of gdt */
+	movl	$apboot_percpu_low, %ecx
+	addl	%eax, %ecx
+	movw	%bx, (%ecx)
+	shr	$16, %ebx
+	addl	$2, %ecx
+	movb	%bl, (%ecx)
+	shr	$8, %bx
+	addl	$3, %ecx
+	movb	%bl, (%ecx)
+
+	/* Patch only our own copy of gdt descriptor */
+	movl	$apboot_gdt_descr, %ecx
+	addl	%eax, %ecx
+	movl	%ecx, %edx
+	addl	$2, %edx
+	movl	(%edx), %ebx
+	addl	%eax, %ebx
+	movl	%ebx, (%edx)
+
+	/* Reload our copy of gdt */
+	lgdtl	(%ecx)
+	ljmpl	$KERNEL_CS, $2f
+2:
 
 	movw	$PERCPU_DS, %ax
 	movw	%ax, %gs
@@ -213,8 +254,10 @@ apboot_jmp_offset:
 	/* Finish the cpu configuration */
 	call EXT(cpu_ap_main)
 
+3:
 	/* NOT REACHED */
 	hlt
+	jmp	3b
 
 .align 16
     .word 0
-- 
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