x86-64/smp: fix random SIGSEGV issues

They seem to have been due to AMD errata 63/122; the fix is to disable
TLB flush filtering in SMP configurations.

Confirmed to fix the problem by Andrew Walrond <andrew@walrond.org>

[ Let's see if we'll have a better fix eventually, this is the Q&D
  "let's get this fixed and out there" version ]
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

arch/x86_64/kernel/setup.c

@@ -831,11 +831,26 @@ static void __init amd_detect_cmp(struct cpuinfo_x86 *c)
 #endif
 }
 
+#define HWCR 0xc0010015
+
 static int __init init_amd(struct cpuinfo_x86 *c)
 {
 	int r;
 	int level;
 
+#ifdef CONFIG_SMP
+	unsigned long value;
+
+	// Disable TLB flush filter by setting HWCR.FFDIS:
+	// bit 6 of msr C001_0015
+	//
+	// Errata 63 for SH-B3 steppings
+	// Errata 122 for all(?) steppings
+	rdmsrl(HWCR, value);
+	value |= 1 << 6;
+	wrmsrl(HWCR, value);
+#endif
+
 	/* Bit 31 in normal CPUID used for nonstandard 3DNow ID;
 	   3DNow is IDd by bit 31 in extended CPUID (1*32+31) anyway */
 	clear_bit(0*32+31, &c->x86_capability);