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[Xen-devel] [Patch V2] expand x86 arch_shared_info to support linear p2m list

The x86 struct arch_shared_info field pfn_to_mfn_frame_list_list
currently contains the mfn of the top level page frame of the 3 level
p2m tree, which is used by the Xen tools during saving and restoring
(and live migration) of pv domains and for crash dump analysis. With
three levels of the p2m tree it is possible to support up to 512 GB of
RAM for a 64 bit pv domain.

A 32 bit pv domain can support more, as each memory page can hold 1024
instead of 512 entries, leading to a limit of 4 TB.

To be able to support more RAM on x86-64 switch to a virtual mapped
p2m list.

This patch expands struct arch_shared_info with a new p2m list virtual
address, the root of the page table root and a p2m generation count.
The new information is indicated by the domain to be valid by storing
~0UL into pfn_to_mfn_frame_list_list. The hypervisor indicates
usability of this feature by a new flag XENFEAT_virtual_p2m.

Right now XENFEAT_virtual_p2m will not be set. This will change when
the Xen tools support the virtual mapped p2m list.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
 xen/include/public/arch-x86/xen.h | 22 +++++++++++++++++++++-
 xen/include/public/features.h     |  3 +++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/xen/include/public/arch-x86/xen.h 
index f35804b..14d3090 100644
--- a/xen/include/public/arch-x86/xen.h
+++ b/xen/include/public/arch-x86/xen.h
@@ -224,7 +224,27 @@ struct arch_shared_info {
     /* Frame containing list of mfns containing list of mfns containing p2m. */
     xen_pfn_t     pfn_to_mfn_frame_list_list;
     unsigned long nmi_reason;
-    uint64_t pad[32];
+    /*
+     * Following three fields are valid if pfn_to_mfn_frame_list_list contains
+     * ~0UL.
+     * p2m_vaddr holds the virtual address of the linear p2m list. All entries
+     * in the range [0...max_pfn[ are accessible via this pointer.
+     * p2m_cr3 is the root of the address space where p2m_vaddr is valid.
+     * p2m_cr3 is in the same format as a cr3 value in the vcpu register state
+     * and holds the folded machine frame number (via xen_pfn_to_cr3) of a
+     * L3 or L4 page table.
+     * p2m_generation will be incremented by the guest before and after each
+     * change of the mappings of the p2m list. p2m_generation starts at 0 and
+     * a value with the least significant bit set indicates that a mapping
+     * update is in progress. This allows guest external software (e.g. in 
+     * to verify that read mappings are consistent and whether they have 
+     * since the last check.
+     * Modifying a p2m element in the linear p2m list is allowed via an atomic
+     * write only.
+     */
+    unsigned long p2m_vaddr;       /* virtual address of the p2m list */
+    unsigned long p2m_cr3;         /* cr3 value of the p2m address space */
+    unsigned long p2m_generation;  /* generation count of p2m mapping */
 typedef struct arch_shared_info arch_shared_info_t;
diff --git a/xen/include/public/features.h b/xen/include/public/features.h
index 16d92aa..ff0b82d 100644
--- a/xen/include/public/features.h
+++ b/xen/include/public/features.h
@@ -99,6 +99,9 @@
 #define XENFEAT_grant_map_identity        12
+/* x86: guest may specify virtual address of p2m list */
+#define XENFEAT_virtual_p2m               13
 #endif /* __XEN_PUBLIC_FEATURES_H__ */

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