Handle spurious page faults that may occur in no-fault sections of the

kernel.

When access restrictions are added to a page table entry, we flush the
corresponding virtual address mapping from the TLB.  In contrast, when
access restrictions are removed from a page table entry, we do not
flush the virtual address mapping from the TLB.  This is exactly as
recommended in AMD's documentation.  In effect, when access
restrictions are removed from a page table entry, AMD's MMUs will
transparently refresh a stale TLB entry.  In short, this saves us from
having to perform potentially costly TLB flushes.  In contrast,
Intel's MMUs are allowed to generate a spurious page fault based upon
the stale TLB entry.  Usually, such spurious page faults are handled
by vm_fault() without incident.  However, when we are executing
no-fault sections of the kernel, we are not allowed to execute
vm_fault().  This change introduces special-case handling for spurious
page faults that occur in no-fault sections of the kernel.

In collaboration with:	kib
Tested by:		gibbs (an earlier version)

I would also like to acknowledge Hiroki Sato's assistance in
diagnosing this problem.

MFC after:	1 week

1 files changed, 7 insertions, 1 deletions

diff --git a/sys/vm/vm_fault.c b/sys/vm/vm_fault.c
index 8c98b26..8477c35 100644
--- a/sys/vm/vm_fault.c
+++ b/sys/vm/vm_fault.c
@@ -1468,11 +1468,17 @@ vm_fault_additional_pages(m, rbehind, rahead, marray, reqpage)
 	return i;
 }
 
+/*
+ * Block entry into the machine-independent layer's page fault handler by
+ * the calling thread.  Subsequent calls to vm_fault() by that thread will
+ * return KERN_PROTECTION_FAILURE.  Enable machine-dependent handling of
+ * spurious page faults. 
+ */
 int
 vm_fault_disable_pagefaults(void)
 {
 
-	return (curthread_pflags_set(TDP_NOFAULTING));
+	return (curthread_pflags_set(TDP_NOFAULTING | TDP_RESETSPUR));
 }
 
 void