x86/boot: Simplify the GDTR calculation assembly code a bit

This patch calculates the GDTR's base address via a single instruction.

( EBP contains the address where it is loaded and GDTR's base address is
  already set to "gdt" in compilation. It is fine to get the correct base
  address by adding the delta to GDTR's base address. )

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1478015364-5547-1-git-send-email-richard.weiyang@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index efdfba2..4d85e60 100644 (file)
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -119,8 +119,7 @@ ENTRY(startup_32)
  */
 
        /* Load new GDT with the 64bit segments using 32bit descriptor */
-       leal    gdt(%ebp), %eax
-       movl    %eax, gdt+2(%ebp)
+       addl    %ebp, gdt+2(%ebp)
        lgdt    gdt(%ebp)
 
        /* Enable PAE mode */