[Xen-changelog] [xen-unstable] [XEN] Do not emulate user-mode updates to page tables.

[Xen patchbot-unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User kfraser@xxxxxxxxxxxxxxxxxxxxx
# Node ID 6e3223a6946142eab3fb14df8fde5c817c53a44b
# Parent  8c29bf7ede67044bc8364e7a5c203b90281f6fd6
[XEN] Do not emulate user-mode updates to page tables.
Instead use as a hint that the page is no longer a page table.
Signed-off-by: Keir Fraser <keir@xxxxxxxxxxxxx>
---
 xen/arch/x86/mm/shadow/multi.c |   14 ++++++++++----
 1 files changed, 10 insertions(+), 4 deletions(-)

diff -r 8c29bf7ede67 -r 6e3223a69461 xen/arch/x86/mm/shadow/multi.c
--- a/xen/arch/x86/mm/shadow/multi.c    Thu Sep 21 19:34:45 2006 +0100
+++ b/xen/arch/x86/mm/shadow/multi.c    Thu Sep 21 19:51:10 2006 +0100
@@ -3037,7 +3037,14 @@ static int sh_page_fault(struct vcpu *v,
     SHADOW_PRINTK("emulate: eip=%#lx\n", emul_regs.eip);
 
     v->arch.shadow.propagate_fault = 0;
-    if ( x86_emulate_memop(&emul_ctxt, &shadow_emulator_ops) )
+
+    /*
+     * We do not emulate user writes. Instead we use them as a hint that the
+     * page is no longer a page table. This behaviour differs from native, but
+     * it seems very unlikely that any OS grants user access to page tables.
+     */
+    if ( (regs->error_code & PFEC_user_mode) ||
+         x86_emulate_memop(&emul_ctxt, &shadow_emulator_ops) )
     {
         SHADOW_PRINTK("emulator failure, unshadowing mfn %#lx\n", 
                        mfn_x(gmfn));
@@ -3050,11 +3057,10 @@ static int sh_page_fault(struct vcpu *v,
          * guest to loop on the same page fault. */
         goto done;
     }
+
+    /* Emulation triggered another page fault? */
     if ( v->arch.shadow.propagate_fault )
-    {
-        /* Emulation triggered another page fault */
         goto not_a_shadow_fault;
-    }
 
     /* Emulator has changed the user registers: write back */
     if ( hvm_guest(v) )

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