xen/setup: Remap Xen Identity Mapped RAM

Instead of ballooning up and down dom0 memory this remaps the existing mfns
that were replaced by the identity map. The reason for this is that the
existing implementation ballooned memory up and and down which caused dom0
to have discontiguous pages. In some cases this resulted in the use of bounce
buffers which reduced network I/O performance significantly. This change will
honor the existing order of the pages with the exception of some boundary
conditions.

To do this we need to update both the Linux p2m table and the Xen m2p table.
Particular care must be taken when updating the p2m table since it's important
to limit table memory consumption and reuse the existing leaf pages which get
freed when an entire leaf page is set to the identity map. To implement this,
mapping updates are grouped into blocks with table entries getting cached
temporarily and then released.

On my test system before:
Total pages: 2105014
Total contiguous: 1640635

After:
Total pages: 2105014
Total contiguous: 2098904
Signed-off-by: Matthew Rushton <mrushton@amazon.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

arch/x86/xen/p2m.c

@@ -173,6 +173,7 @@
 #include <xen/balloon.h>
 #include <xen/grant_table.h>
 
+#include "p2m.h"
 #include "multicalls.h"
 #include "xen-ops.h"
 
@@ -180,12 +181,6 @@ static void __init m2p_override_init(void);
 
 unsigned long xen_max_p2m_pfn __read_mostly;
 
-#define P2M_PER_PAGE		(PAGE_SIZE / sizeof(unsigned long))
-#define P2M_MID_PER_PAGE	(PAGE_SIZE / sizeof(unsigned long *))
-#define P2M_TOP_PER_PAGE	(PAGE_SIZE / sizeof(unsigned long **))
-
-#define MAX_P2M_PFN		(P2M_TOP_PER_PAGE * P2M_MID_PER_PAGE * P2M_PER_PAGE)
-
 /* Placeholders for holes in the address space */
 static RESERVE_BRK_ARRAY(unsigned long, p2m_missing, P2M_PER_PAGE);
 static RESERVE_BRK_ARRAY(unsigned long *, p2m_mid_missing, P2M_MID_PER_PAGE);
@@ -202,16 +197,12 @@ static RESERVE_BRK_ARRAY(unsigned long, p2m_mid_identity_mfn, P2M_MID_PER_PAGE);
 RESERVE_BRK(p2m_mid, PAGE_SIZE * (MAX_DOMAIN_PAGES / (P2M_PER_PAGE * P2M_MID_PER_PAGE)));
 RESERVE_BRK(p2m_mid_mfn, PAGE_SIZE * (MAX_DOMAIN_PAGES / (P2M_PER_PAGE * P2M_MID_PER_PAGE)));
 
-/* We might hit two boundary violations at the start and end, at max each
- * boundary violation will require three middle nodes. */
-RESERVE_BRK(p2m_mid_extra, PAGE_SIZE * 2 * 3);
-
-/* When we populate back during bootup, the amount of pages can vary. The
- * max we have is seen is 395979, but that does not mean it can't be more.
- * Some machines can have 3GB I/O holes even. With early_can_reuse_p2m_middle
- * it can re-use Xen provided mfn_list array, so we only need to allocate at
- * most three P2M top nodes. */
-RESERVE_BRK(p2m_populated, PAGE_SIZE * 3);
+/* For each I/O range remapped we may lose up to two leaf pages for the boundary
+ * violations and three mid pages to cover up to 3GB. With
+ * early_can_reuse_p2m_middle() most of the leaf pages will be reused by the
+ * remapped region.
+ */
+RESERVE_BRK(p2m_identity_remap, PAGE_SIZE * 2 * 3 * MAX_REMAP_RANGES);
 
 static inline unsigned p2m_top_index(unsigned long pfn)
 {

arch/x86/xen/p2m.h

+#ifndef _XEN_P2M_H
+#define _XEN_P2M_H
+
+#define P2M_PER_PAGE        (PAGE_SIZE / sizeof(unsigned long))
+#define P2M_MID_PER_PAGE    (PAGE_SIZE / sizeof(unsigned long *))
+#define P2M_TOP_PER_PAGE    (PAGE_SIZE / sizeof(unsigned long **))
+
+#define MAX_P2M_PFN         (P2M_TOP_PER_PAGE * P2M_MID_PER_PAGE * P2M_PER_PAGE)
+
+#define MAX_REMAP_RANGES    10
+
+extern unsigned long __init set_phys_range_identity(unsigned long pfn_s,
+                                      unsigned long pfn_e);
+
+#endif  /* _XEN_P2M_H */