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 ---
Changes in v3: None Changes in v2: - Add a comment about the REX prefix - Drop the REX prefix in 32-bit mode
arch/x86/cpu/Makefile | 6 +++++ arch/x86/cpu/call32.S | 64 ++++++++++++++++++++++++++++++++++++++++++++++ arch/x86/include/asm/cpu.h | 9 +++++++ 3 files changed, 79 insertions(+) create mode 100644 arch/x86/cpu/call32.S
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 @@ 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 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 @@ 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 * * The kernel is uncompressed and the 64-bit entry point is expected to be