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 5e058c0c097849a001b23dfc42479cfc2486c35b..e7979254dbfad255366d35d68f397ee4538d8e81 100644 (file)
--- a/arch/x86/cpu/Makefile
+++ b/arch/x86/cpu/Makefile
@@ -12,6 +12,12 @@ extra-y      = start.o
 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 (file)
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 5b891394fb6a6af65a7bef8f8a9d11e0d7333ee5..c70183ccefffba8effd9fdb080210328285b03a3 100644 (file)
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -229,6 +229,15 @@ char *cpu_get_name(char *name);
  */
 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
  *