x86/boot/compressed/64: Setup IDT in startup_32 boot path

This boot path needs exception handling when it is used with SEV-ES.
Setup an IDT and provide a helper function to write IDT entries for
use in 32-bit protected mode.
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210312123824.306-5-joro@8bytes.org

arch/x86/boot/compressed/head_64.S

@@ -116,6 +116,9 @@ SYM_FUNC_START(startup_32)
 	lretl
 1:
 
+	/* Setup Exception handling for SEV-ES */
+	call	startup32_load_idt
+
 	/* Make sure cpu supports long mode. */
 	call	verify_cpu
 	testl	%eax, %eax
@@ -701,6 +704,19 @@ SYM_DATA_START(boot_idt)
 	.endr
 SYM_DATA_END_LABEL(boot_idt, SYM_L_GLOBAL, boot_idt_end)
 
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+SYM_DATA_START(boot32_idt_desc)
+	.word   boot32_idt_end - boot32_idt - 1
+	.long   0
+SYM_DATA_END(boot32_idt_desc)
+	.balign 8
+SYM_DATA_START(boot32_idt)
+	.rept 32
+	.quad 0
+	.endr
+SYM_DATA_END_LABEL(boot32_idt, SYM_L_GLOBAL, boot32_idt_end)
+#endif
+
 #ifdef CONFIG_EFI_STUB
 SYM_DATA(image_offset, .long 0)
 #endif
@@ -793,6 +809,62 @@ SYM_DATA_START_LOCAL(loaded_image_proto)
 SYM_DATA_END(loaded_image_proto)
 #endif
 
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+	__HEAD
+	.code32
+/*
+ * Write an IDT entry into boot32_idt
+ *
+ * Parameters:
+ *
+ * %eax:	Handler address
+ * %edx:	Vector number
+ *
+ * Physical offset is expected in %ebp
+ */
+SYM_FUNC_START(startup32_set_idt_entry)
+	push    %ebx
+	push    %ecx
+
+	/* IDT entry address to %ebx */
+	leal    rva(boot32_idt)(%ebp), %ebx
+	shl	$3, %edx
+	addl    %edx, %ebx
+
+	/* Build IDT entry, lower 4 bytes */
+	movl    %eax, %edx
+	andl    $0x0000ffff, %edx	# Target code segment offset [15:0]
+	movl    $__KERNEL32_CS, %ecx	# Target code segment selector
+	shl     $16, %ecx
+	orl     %ecx, %edx
+
+	/* Store lower 4 bytes to IDT */
+	movl    %edx, (%ebx)
+
+	/* Build IDT entry, upper 4 bytes */
+	movl    %eax, %edx
+	andl    $0xffff0000, %edx	# Target code segment offset [31:16]
+	orl     $0x00008e00, %edx	# Present, Type 32-bit Interrupt Gate
+
+	/* Store upper 4 bytes to IDT */
+	movl    %edx, 4(%ebx)
+
+	pop     %ecx
+	pop     %ebx
+	ret
+SYM_FUNC_END(startup32_set_idt_entry)
+#endif
+
+SYM_FUNC_START(startup32_load_idt)
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+	/* Load IDT */
+	leal	rva(boot32_idt)(%ebp), %eax
+	movl	%eax, rva(boot32_idt_desc+2)(%ebp)
+	lidt    rva(boot32_idt_desc)(%ebp)
+#endif
+	ret
+SYM_FUNC_END(startup32_load_idt)
+
 /*
  * Stack and heap for uncompression
  */