[PATCH] x86: Cr4 is valid on some 486s

So some 486 processors do have CR4 register.  Allow them to present it in
register dumps by using the old fault technique rather than testing processor
family.

Thanks to Maciej for noticing this.
Signed-off-by: NZachary Amsden <zach@vmware.com>
Cc: "Seth, Rohit" <rohit.seth@intel.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

arch/i386/kernel/process.c

@@ -308,9 +308,7 @@ void show_regs(struct pt_regs * regs)
 	cr0 = read_cr0();
 	cr2 = read_cr2();
 	cr3 = read_cr3();
-	if (current_cpu_data.x86 > 4) {
-		cr4 = read_cr4();
-	}
+	cr4 = read_cr4_safe();
 	printk("CR0: %08lx CR2: %08lx CR3: %08lx CR4: %08lx\n", cr0, cr2, cr3, cr4);
 	show_trace(NULL, &regs->esp);
 }

include/asm-i386/system.h

@@ -140,6 +140,19 @@ static inline unsigned long _get_base(char * addr)
 		:"=r" (__dummy)); \
 	__dummy; \
 })
+
+#define read_cr4_safe() ({			      \
+	unsigned int __dummy;			      \
+	/* This could fault if %cr4 does not exist */ \
+	__asm__("1: movl %%cr4, %0		\n"   \
+		"2:				\n"   \
+		".section __ex_table,\"a\"	\n"   \
+		".long 1b,2b			\n"   \
+		".previous			\n"   \
+		: "=r" (__dummy): "0" (0));	      \
+	__dummy;				      \
+})
+
 #define write_cr4(x) \
 	__asm__ __volatile__("movl %0,%%cr4": :"r" (x));
 #define stts() write_cr0(8 | read_cr0())