Linux Virtualization - Apr 2007 - [PATCH 5/14] i386 / Use early clobber to eliminate rotate in desc

Use an early clobber on addr to avoid the extra rorl instruction at the
end of _set_tssldt_desc.

Also, get some C type checking on the descriptor struct here.

Patch-base: 2.6.13-rc5-mm1
Patch-keys: i386 desc cleanup optimize
Signed-off-by: Zachary Amsden <zach@vmware.com>
Index: linux-2.6.13/include/asm-i386/desc.h
==================================================================---
linux-2.6.13.orig/include/asm-i386/desc.h	2005-08-09 18:59:10.000000000 -0700
+++ linux-2.6.13/include/asm-i386/desc.h	2005-08-10 20:42:20.000000000 -0700
@@ -34,17 +34,21 @@
 extern struct desc_struct default_ldt[];
 extern void set_intr_gate(unsigned int irq, void * addr);
 
-#define _set_tssldt_desc(n,addr,limit,type) \
-__asm__ __volatile__ ("movw %w3,0(%2)\n\t" \
-	"movw %w1,2(%2)\n\t" \
-	"rorl $16,%1\n\t" \
-	"movb %b1,4(%2)\n\t" \
-	"movb %4,5(%2)\n\t" \
-	"movb $0,6(%2)\n\t" \
-	"movb %h1,7(%2)\n\t" \
-	"rorl $16,%1" \
-	: "=m"(*(n)) : "q" (addr), "r"(n),
"ir"(limit), "i"(type))
-
+#define _set_tssldt_desc(desc,addr,limit,type) \
+do { \
+	unsigned long __tmp; \
+	typecheck(struct desc_struct *, desc); \
+	asm volatile ("movw %w4,0(%3)\n\t" \
+		      "movw %w2,2(%3)\n\t" \
+		      "rorl $16,%2\n\t" \
+		      "movb %b2,4(%3)\n\t" \
+		      "movb %5,5(%3)\n\t" \
+		      "movb $0,6(%3)\n\t" \
+		      "movb %h2,7(%3)\n\t" \
+		      : "=m"(*(desc)), "=&q" (__tmp) \
+		      : "1" (addr), "r"(desc), "ir"(limit),
"i"(type)); \
+} while (0)
+  
 #include <mach_desc.h>
 
 #define set_tss_desc(cpu,addr) __set_tss_desc(cpu, GDT_ENTRY_TSS, addr)

On Wed, Aug 10, 2005 at 09:54:11PM -0700, zach@vmware.com
wrote:
> Use an early clobber on addr to avoid the extra rorl instruction at the
> end of _set_tssldt_desc.
I would suggest to just use C for this. I do this on x86-64 and 
I don't think there is any reason to use this hard to maintain
code for it.

It's probably a left over from Linus first experiments with inline
assembly, similar to the old string.h and by now so obsolete
it doesn't even stink anymore.

-Andi