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Subject[PATCH 03 of 12] xen: efficiently support a holey p2m table
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When using sparsemem and memory hotplug, the kernel's pseudo-physical
address space can be discontigious. Previously this was dealt with by
having the upper parts of the radix tree stubbed off. Unfortunately,
this is incompatible with save/restore, which requires a complete p2m
table.

The solution is to have a special distinguished all-invalid p2m leaf
page, which we can point all the hole areas at. This allows the tools
to see a complete p2m table, but it only costs a page for all memory
holes.

It also simplifies the code since it removes a few special cases.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
---
arch/x86/xen/mmu.c | 18 ++++++++++++------
1 file changed, 12 insertions(+), 6 deletions(-)
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -57,8 +57,17 @@
#include "mmu.h"

#define P2M_ENTRIES_PER_PAGE (PAGE_SIZE / sizeof(unsigned long))
+#define TOP_ENTRIES (MAX_DOMAIN_PAGES / P2M_ENTRIES_PER_PAGE)

-static unsigned long *p2m_top[MAX_DOMAIN_PAGES / P2M_ENTRIES_PER_PAGE];
+/* Placeholder for holes in the address space */
+static unsigned long p2m_missing[P2M_ENTRIES_PER_PAGE]
+ __attribute__((section(".data.page_aligned"))) =
+ { [ 0 ... P2M_ENTRIES_PER_PAGE-1 ] = ~0UL };
+
+ /* Array of pointers to pages containing p2m entries */
+static unsigned long *p2m_top[TOP_ENTRIES]
+ __attribute__((section(".data.page_aligned"))) =
+ { [ 0 ... TOP_ENTRIES - 1] = &p2m_missing[0] };

static inline unsigned p2m_top_index(unsigned long pfn)
{
@@ -92,9 +101,6 @@
return INVALID_P2M_ENTRY;

topidx = p2m_top_index(pfn);
- if (p2m_top[topidx] == NULL)
- return INVALID_P2M_ENTRY;
-
idx = p2m_index(pfn);
return p2m_top[topidx][idx];
}
@@ -110,7 +116,7 @@
for(i = 0; i < P2M_ENTRIES_PER_PAGE; i++)
p[i] = INVALID_P2M_ENTRY;

- if (cmpxchg(pp, NULL, p) != NULL)
+ if (cmpxchg(pp, p2m_missing, p) != p2m_missing)
free_page((unsigned long)p);
}

@@ -129,7 +135,7 @@
}

topidx = p2m_top_index(pfn);
- if (p2m_top[topidx] == NULL) {
+ if (p2m_top[topidx] == p2m_missing) {
/* no need to allocate a page to store an invalid entry */
if (mfn == INVALID_P2M_ENTRY)
return;



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