x86: Add a way to call 32-bit code from 64-bit mode
The procedure to drop from 64-bit mode to 32-bit is a bit messy. Add a
function to take care of it. It requires identity-mapped pages and that
the calling code is running below 4GB.
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/x86/cpu/call32.S b/arch/x86/cpu/call32.S
new file mode 100644
index 0000000..c517e4a
--- /dev/null
+++ b/arch/x86/cpu/call32.S
@@ -0,0 +1,64 @@
+/*
+ * (C) Copyright 2015 Google, Inc
+ * Written by Simon Glass <sjg@chromium.org>
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ */
+
+#include <asm/global_data.h>
+#include <asm/msr-index.h>
+#include <asm/processor-flags.h>
+
+ /*
+ * rdi - 32-bit code segment selector
+ * rsi - target address
+ * rdx - table address (0 if none)
+ */
+.code64
+.globl cpu_call32
+cpu_call32:
+ cli
+
+ /* Save table pointer */
+ mov %edx, %ebx
+
+ /*
+ * Debugging option, this outputs characters to the console UART
+ * mov $0x3f8,%edx
+ * mov $'a',%al
+ * out %al,(%dx)
+ */
+
+ pushf
+ push %rdi /* 32-bit code segment */
+ lea compat(%rip), %rax
+ push %rax
+ .byte 0x48 /* REX prefix to force 64-bit far return */
+ retf
+.code32
+compat:
+ /*
+ * We are now in compatibility mode with a default operand size of
+ * 32 bits. First disable paging.
+ */
+ movl %cr0, %eax
+ andl $~X86_CR0_PG, %eax
+ movl %eax, %cr0
+
+ /* Invalidate TLB */
+ xorl %eax, %eax
+ movl %eax, %cr3
+
+ /* Disable Long mode in EFER (Extended Feature Enable Register) */
+ movl $MSR_EFER, %ecx
+ rdmsr
+ btr $_EFER_LME, %eax
+ wrmsr
+
+ /* Set up table pointer for _x86boot_start */
+ mov %ebx, %ecx
+
+ /* Jump to the required target */
+ pushl %edi /* 32-bit code segment */
+ pushl %esi /* 32-bit target address */
+ retf