[PATCH] Work around K8 errata on x86-64

K8 has an erratum (#100) that essentially causes some compat mode processes
to fault occassionally. The issue can be worked around in the OS. It only
applies to x86-64, in 32bit it is fine.

This adds a check to the page fault handler that checks for addresses >4GB
from compat mode. If they happen just return; the CPU will reexecute
the instruction and the condition that caused the problem is gone.

More details in Opteron/Athlon64 specification update on the AMD website.

Also I removed a left over debugging printk in the prefetch handling code.

arch/x86_64/mm/fault.c

@@ -126,12 +126,6 @@ static int is_prefetch(struct pt_regs *regs, unsigned long addr)
 			break;
 		} 
 	}
-
-#if 1
-	if (prefetch)
-		printk("%s: prefetch caused page fault at %lx/%lx\n", current->comm,
-		       regs->rip, addr);
-#endif
 	return prefetch;
 }
 
@@ -241,6 +235,15 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long error_code)
 	if (unlikely(in_atomic() || !mm))
 		goto bad_area_nosemaphore;
 
+	/* Work around K8 erratum #100
+	   K8 in compat mode occasionally jumps to illegal addresses >4GB.
+	   We catch this here in the page fault handler because these
+	   addresses are not reachable. Just detect this case and return.
+	   Any code segment in LDT is compatibility mode. */
+	if ((regs->cs == __USER32_CS || (regs->cs & (1<<2))) &&
+		(address >> 32))
+		return;
+
  again:
 	down_read(&mm->mmap_sem);