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, 1 insertions, 0 deletions

diff --git a/sys/amd64/include/proc.h b/sys/amd64/include/proc.h
index 5207d89..14585fb 100644
--- a/sys/amd64/include/proc.h
+++ b/sys/amd64/include/proc.h
@@ -46,6 +46,7 @@ struct proc_ldt {
 struct mdthread {
 	int	md_spinlock_count;	/* (k) */
 	register_t md_saved_flags;	/* (k) */
+	register_t md_spurflt_addr;	/* (k) Spurious page fault address. */
 };
 
 struct mdproc {