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Re: [Xen-devel] WP flag in CR0, setting

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] WP flag in CR0, setting
From: Simon Kagstrom <simon.kagstrom@xxxxxx>
Date: Mon, 03 Jul 2006 13:50:10 +0200
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At Thu, 15 Jun 2006 09:08:37 +0100,
Keir Fraser wrote:
> On 15 Jun 2006, at 09:00, Simon Kagstrom wrote:
> > Thanks for the explanation. Would the WP flag work in a HVM setting?
> It may not be supported right now, but it would be easy to do so if not.

Does Xen itself rely on running with the WP flag set? I'll go with the
HVM supported WP-bit later, but as I don't have the hardware yet, I
wanted to try to just clear the bit in Xen, and that seems to cause
problems. Changing

diff -r 17fbffb737d9 xen/arch/x86/boot/x86_32.S
--- a/xen/arch/x86/boot/x86_32.S        Thu Jun 22 21:41:19 2006 +0100
+++ b/xen/arch/x86/boot/x86_32.S        Mon Jul 03 13:43:07 2006 +0200
@@ -150,7 +150,7 @@ no_execute_disable:
         mov     $idle_pg_table-__PAGE_OFFSET,%eax
         mov     %eax,%cr3
-        mov     $0x80050033,%eax /* hi-to-lo: PG,AM,WP,NE,ET,MP,PE */
+        mov     $0x80040033,%eax /* hi-to-lo: PG,AM,NE,ET,MP,PE */
         mov     %eax,%cr0
         jmp     1f
 1:      /* Install relocated selectors (FS/GS unused). */

Causes an assertion to trigger when booting the Linux dom0:

(XEN) Assertion '(x & PGT_count_mask) != 0' failed, line 1430, file mm.c

Is this due to how Xen works or is it because Linux uses writable page
tables (I have not checked the Linux sources other than the

// Simon

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