[PATCH 31/74] x86/head/64: Install boot GDT

[Joerg Roedel]

Handling exceptions during boot requires a working GDT. The kernel GDT
is not yet ready for use, so install a temporary boot GDT.

Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
 arch/x86/kernel/head64.c  | 19 +++++++++++++++++++
 arch/x86/kernel/head_64.S | 25 +++++++++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 206a4b6144c2..900c3372cc6a 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -61,6 +61,25 @@ unsigned long vmemmap_base __ro_after_init = __VMEMMAP_BASE_L4;
 EXPORT_SYMBOL(vmemmap_base);
 #endif
 
+/*
+ * GDT used before %gs is set up and the kernel can use gdt_page. Needed for
+ * early exception handling.
+ */
+struct desc_struct boot_gdt[GDT_ENTRIES] = {
+	[GDT_ENTRY_KERNEL32_CS]         = GDT_ENTRY_INIT(0xc09b, 0, 0xfffff),
+	[GDT_ENTRY_KERNEL_CS]           = GDT_ENTRY_INIT(0xa09b, 0, 0xfffff),
+	[GDT_ENTRY_KERNEL_DS]           = GDT_ENTRY_INIT(0xc093, 0, 0xfffff),
+};
+
+/*
+ * Address needs to be set at runtime because it references the boot_gdt while
+ * the kernel still uses a direct mapping.
+ */
+struct desc_ptr boot_gdt_descr = {
+	.size = sizeof(boot_gdt),
+	.address = 0,
+};
+
 #define __head	__section(.head.text)
 
 static void __head *fixup_pointer(void *ptr, unsigned long physaddr)
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 4bbc770af632..62513dd1e0e4 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -72,6 +72,26 @@ SYM_CODE_START_NOALIGN(startup_64)
 	/* Set up the stack for verify_cpu(), similar to initial_stack below */
 	leaq	(__end_init_task - SIZEOF_PTREGS)(%rip), %rsp
 
+	/* Setup boot GDT descriptor and load boot GDT */
+	leaq	boot_gdt(%rip), %rax
+	movq	%rax, boot_gdt_descr+2(%rip)
+	lgdt	boot_gdt_descr(%rip)
+
+	/* New GDT is live - reload data segment registers */
+	movl	$__KERNEL_DS, %eax
+	movl	%eax, %ds
+	movl	%eax, %ss
+	movl	%eax, %es
+
+	/* Now switch to __KERNEL_CS so IRET works reliably */
+	pushq	$__KERNEL_CS
+	leaq	.Lon_kernel_cs(%rip), %rax
+	pushq	%rax
+	lretq
+
+.Lon_kernel_cs:
+	UNWIND_HINT_EMPTY
+
 	/* Sanitize CPU configuration */
 	call verify_cpu
 
@@ -143,6 +163,11 @@ SYM_CODE_START(secondary_startup_64)
 1:
 	UNWIND_HINT_EMPTY
 
+	/* Setup boot GDT descriptor and load boot GDT */
+	leaq	boot_gdt(%rip), %rax
+	movq	%rax, boot_gdt_descr+2(%rip)
+	lgdt	boot_gdt_descr(%rip)
+
 	/* Check if nx is implemented */
 	movl	$0x80000001, %eax
 	cpuid
-- 
2.26.2