[PATCH]: Make linux smarter about using batched PTE updates

[Chris Lalancette <clalance@redhat.com>]

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All,
     Attached is a patch to make batched PTE updates smarter.  Basically, if you
run a kernel with batched PTE capability on a hypervisor that doesn't support
it, you get lots of:

(XEN) mm.c:2453:d2 Invalid page update command 2
(XEN) printk: 333 messages suppressed.
(XEN) mm.c:2453:d2 Invalid page update command 2

on the hypervisor console.

It doesn't seem to be a security problem, since it is rate-limited, but it is an
annoyance.  The patch just looks at the return code from the first such
hypercall, and if we fail with ENOSYS, we just mark it as
"hypervisor_no_batch_update" and never try again.  Patch originally from Rik van
Riel.

Signed-off-by: Chris Lalancette <clalance@redhat.com>

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Date: Tue, 11 Mar 2008 16:31:08 -0400
From: Rik van Riel <riel@redhat.com>
To: clalance@redhat.com
Subject: quiet down xen mprotect printks
Message-ID: <20080311163108.0d6ae553@cuia.boston.redhat.com>
Organization: Red Hat, Inc
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It doesn't get much simpler than this :)

Please double-check the bit in mm/mmap.c to make sure I did this the right way around.

--- linux-2.6.18.noarch/arch/i386/mm/hypervisor.c.printk	2008-03-11 18:12:34.000000000 -0400
+++ linux-2.6.18.noarch/arch/i386/mm/hypervisor.c	2008-03-11 18:16:38.000000000 -0400
@@ -458,6 +458,7 @@
 #endif
 
 #define MAX_BATCHED_FULL_PTES 32
+static int hypervisor_no_batch_update;
 
 int xen_change_pte_range(struct mm_struct *mm, pmd_t *pmd,
 			 unsigned long addr, unsigned long end, pgprot_t newprot)
@@ -467,6 +468,9 @@
 	pte_t *pte;
 	spinlock_t *ptl;
 
+	if (hypervisor_no_batch_update)
+		return 0;
+
 	pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
 	do {
 		if (pte_present(*pte)) {
@@ -484,5 +488,8 @@
 		rc = HYPERVISOR_mmu_update( &u[0], i, NULL, DOMID_SELF);
 	pte_unmap_unlock(pte - 1, ptl);
 	BUG_ON(rc && rc != -ENOSYS);
+	/* Don't try again. This hypervisor does not support batch updates. */
+	if (rc == -ENOSYS)
+		hypervisor_no_batch_update = 1;
 	return !rc;
 }

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