x86 setup: make PM transition more paranoid; cleanup 32-bit entry

Make the transition to protected mode more paranoid by having
back-to-back near jump (to synchronize the 386/486 prefetch queue) and
far jump (to set up the code segment.)

While we're at it, zero as many registers as practical (for future
expandability of the 32-bit entry interface) and enter 32-bit mode
with a valid stack.  Note that the 32-bit code cannot rely on this
stack, or we'll break all other existing users of the 32-bit
entrypoint, but it may make debugging hacks easier to write.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/arch/x86/boot/pmjump.S b/arch/x86/boot/pmjump.S
index fa6bed1..ef0da1f 100644 (file)
--- a/arch/x86/boot/pmjump.S
+++ b/arch/x86/boot/pmjump.S
@@ -29,12 +29,13 @@
  */
 protected_mode_jump:
        movl    %edx, %esi              # Pointer to boot_params table
-       movl    %eax, 2f                # Patch ljmpl instruction
+
+       xorl    %ebx, %ebx
+       movw    %cs, %bx
+       shll    $4, %ebx
+       addl    %ebx, 2f
 
        movw    $__BOOT_DS, %cx
-       xorl    %ebx, %ebx              # Per the 32-bit boot protocol
-       xorl    %ebp, %ebp              # Per the 32-bit boot protocol
-       xorl    %edi, %edi              # Per the 32-bit boot protocol
 
        movl    %cr0, %edx
        orb     $1, %dl                 # Protected mode (PE) bit
@@ -42,15 +43,34 @@ protected_mode_jump:
        jmp     1f                      # Short jump to serialize on 386/486
 1:
 
-       movw    %cx, %ds
-       movw    %cx, %es
-       movw    %cx, %fs
-       movw    %cx, %gs
-       movw    %cx, %ss
-
-       # Jump to the 32-bit entrypoint
+       # Transition to 32-bit mode
        .byte   0x66, 0xea              # ljmpl opcode
-2:     .long   0                       # offset
+2:     .long   in_pm32                 # offset
        .word   __BOOT_CS               # segment
 
        .size   protected_mode_jump, .-protected_mode_jump
+
+       .code32
+       .type   in_pm32, @function
+in_pm32:
+       # Set up data segments for flat 32-bit mode
+       movl    %ecx, %ds
+       movl    %ecx, %es
+       movl    %ecx, %fs
+       movl    %ecx, %gs
+       movl    %ecx, %ss
+       # The 32-bit code sets up its own stack, but this way we do have
+       # a valid stack if some debugging hack wants to use it.
+       addl    %ebx, %esp
+
+       # Clear registers to allow for future extensions to the
+       # 32-bit boot protocol
+       xorl    %ecx, %ecx
+       xorl    %edx, %edx
+       xorl    %ebx, %ebx
+       xorl    %ebp, %ebp
+       xorl    %edi, %edi
+
+       jmpl    *%eax                   # Jump to the 32-bit entrypoint
+
+       .size   in_pm32, .-in_pm32