Re: [U-Boot] [PATCH 37/48] x86: Add a way to call 32-bit code from 64-bit mode

Hi Simon,

On Wed, Jul 22, 2015 at 11:49 PM, Simon Glass sjg@chromium.org wrote:

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

arch/x86/cpu/Makefile | 6 +++++ arch/x86/cpu/call32.S | 65 ++++++++++++++++++++++++++++++++++++++++++++++ arch/x86/include/asm/cpu.h | 9 +++++++ 3 files changed, 80 insertions(+) create mode 100644 arch/x86/cpu/call32.S

diff --git a/arch/x86/cpu/Makefile b/arch/x86/cpu/Makefile index 9678976..eb993ec 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_ARCH_EFI) += efi/ diff --git a/arch/x86/cpu/call32.S b/arch/x86/cpu/call32.S new file mode 100644 index 0000000..3fe010e --- /dev/null +++ b/arch/x86/cpu/call32.S @@ -0,0 +1,65 @@ +/*

• • (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
  

I think this is a REX prefix. Is there any compiler macro we can use? We can put some comments here for better understanding. BTW: is this a must? I believe retf will operate on 64-bit by operand in 64-bit by default?

•   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 */
  

•   .byte   0x48
  

•   retf
  

diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h index b96513d..e977045 100644 --- a/arch/x86/include/asm/cpu.h +++ b/arch/x86/include/asm/cpu.h @@ -222,6 +222,15 @@ 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

--

Regards, Bin