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

[Chris Lalancette <clalance@xxxxxxxxxx>]

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@xxxxxxxxxx>

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Date: Tue, 11 Mar 2008 16:31:08 -0400
From: Rik van Riel <riel@xxxxxxxxxx>
To: clalance@xxxxxxxxxx
Subject: quiet down xen mprotect printks
Message-ID: <20080311163108.0d6ae553@xxxxxxxxxxxxxxxxxxxxxx>
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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