x86-64, asm: Directly access per-cpu IST

Use a direct per-cpu reference for the IST instead of using a scratch
register.
Signed-off-by: Brian Gerst <brgerst@gmail.com>
LKML-Reference: <1280594903-6341-1-git-send-email-brgerst@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/kernel/entry_64.S

@@ -1065,6 +1065,7 @@ ENTRY(\sym)
 END(\sym)
 .endm
 
+#define INIT_TSS_IST(x) PER_CPU_VAR(init_tss) + (TSS_ist + ((x) - 1) * 8)
 .macro paranoidzeroentry_ist sym do_sym ist
 ENTRY(\sym)
 	INTR_FRAME
@@ -1076,10 +1077,9 @@ ENTRY(\sym)
 	TRACE_IRQS_OFF
 	movq %rsp,%rdi		/* pt_regs pointer */
 	xorl %esi,%esi		/* no error code */
-	PER_CPU(init_tss, %r12)
-	subq $EXCEPTION_STKSZ, TSS_ist + (\ist - 1) * 8(%r12)
+	subq $EXCEPTION_STKSZ, INIT_TSS_IST(\ist)
 	call \do_sym
-	addq $EXCEPTION_STKSZ, TSS_ist + (\ist - 1) * 8(%r12)
+	addq $EXCEPTION_STKSZ, INIT_TSS_IST(\ist)
 	jmp paranoid_exit	/* %ebx: no swapgs flag */
 	CFI_ENDPROC
 END(\sym)