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/Makefile b/arch/x86/cpu/Makefile
index 5e058c0..e797925 100644
--- a/arch/x86/cpu/Makefile
+++ b/arch/x86/cpu/Makefile
@@ -12,6 +12,12 @@
obj-$(CONFIG_X86_RESET_VECTOR) += resetvec.o start16.o
obj-y += interrupts.o cpu.o cpu_x86.o call64.o
+AFLAGS_REMOVE_call32.o := -mregparm=3 \
+ $(if $(CONFIG_EFI_STUB_64BIT),-march=i386 -m32)
+AFLAGS_call32.o := -fpic -fshort-wchar
+
+extra-y += call32.o
+
obj-$(CONFIG_INTEL_BAYTRAIL) += baytrail/
obj-$(CONFIG_SYS_COREBOOT) += coreboot/
obj-$(CONFIG_EFI_APP) += efi/
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
diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
index 5b89139..c70183c 100644
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -230,6 +230,15 @@
void cpu_call64(ulong pgtable, ulong setup_base, ulong target);
/**
+ * cpu_call32() - Jump to a 32-bit entry point
+ *
+ * @code_seg32: 32-bit code segment to use (GDT offset, e.g. 0x20)
+ * @target: Pointer to the start of the 32-bit U-Boot image/entry point
+ * @table: Pointer to start of info table to pass to U-Boot
+ */
+void cpu_call32(ulong code_seg32, ulong target, ulong table);
+
+/**
* cpu_jump_to_64bit() - Jump to a 64-bit Linux kernel
*
* The kernel is uncompressed and the 64-bit entry point is expected to be