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Re: [Xen-devel] [PATCH 2/2] xen/mm: Introduce PG_state_uninitialised


  • To: <paul@xxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Wed, 18 Mar 2020 11:10:19 +0100
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  • Delivery-date: Wed, 18 Mar 2020 10:19:47 +0000
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On 18.03.2020 09:53, Paul Durrant wrote:
-----Original Message-----
From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of David 
Woodhouse
Sent: 17 March 2020 22:15

On Thu, 2020-02-20 at 12:59 +0100, Jan Beulich wrote:
On 07.02.2020 19:04, David Woodhouse wrote:
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -488,7 +488,8 @@ void share_xen_page_with_guest(struct page_info *page, 
struct domain *d,

      page_set_owner(page, d);
      smp_wmb(); /* install valid domain ptr before updating refcnt. */
-    ASSERT((page->count_info & ~PGC_xen_heap) == 0);
+    ASSERT((page->count_info & ~PGC_xen_heap) == PGC_state_inuse ||
+           (page->count_info & ~PGC_xen_heap) == PGC_state_uninitialised);

Can uninitialized pages really make it here?

Yep, we share the low 1MiB with dom_io.


OOI anyone know why we do this? Is it actually necessary?

Yes, for Dom0 to be able to access things like EBDA, IBFT, or data
found in BIOS space.

Jan

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