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>

1 files changed, 32 insertions, 12 deletions

diff --git a/arch/x86/boot/pmjump.S b/arch/x86/boot/pmjump.S
index fa6bed1fac14..ef0da1f2c7fd 100644
--- 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