xen: add m2p override mechanism

Add a simple hashtable based mechanism to override some portions of the
m2p, so that we can find out the pfn corresponding to an mfn of a
granted page. In fact entries corresponding to granted pages in the m2p
hold the original pfn value of the page in the source domain that
granted it.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

2 files changed, 93 insertions, 3 deletions

diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
index 8760cc60a21c..50f0a0f6bd6a 100644
--- a/arch/x86/include/asm/xen/page.h
+++ b/arch/x86/include/asm/xen/page.h
@@ -42,6 +42,11 @@ extern unsigned int   machine_to_phys_order;
 extern unsigned long get_phys_to_machine(unsigned long pfn);
 extern bool set_phys_to_machine(unsigned long pfn, unsigned long mfn);
 
+extern void m2p_add_override(unsigned long mfn, struct page *page);
+extern void m2p_remove_override(struct page *page);
+extern struct page *m2p_find_override(unsigned long mfn);
+extern unsigned long m2p_find_override_pfn(unsigned long mfn, unsigned long pfn);
+
 static inline unsigned long pfn_to_mfn(unsigned long pfn)
 {
         unsigned long mfn;
@@ -72,9 +77,6 @@ static inline unsigned long mfn_to_pfn(unsigned long mfn)
         if (xen_feature(XENFEAT_auto_translated_physmap))
                 return mfn;
 
-        if (unlikely((mfn >> machine_to_phys_order) != 0))
-                return ~0;
-
         pfn = 0;
         /*
          * The array access can fail (e.g., device space beyond end of RAM).
@@ -83,6 +85,14 @@ static inline unsigned long mfn_to_pfn(unsigned long mfn)
          */
         __get_user(pfn, &machine_to_phys_mapping[mfn]);
 
+        /*
+         * If this appears to be a foreign mfn (because the pfn
+         * doesn't map back to the mfn), then check the local override
+         * table to see if there's a better pfn to use.
+         */
+        if (get_phys_to_machine(pfn) != mfn)
+                pfn = m2p_find_override_pfn(mfn, pfn);
+
         return pfn;
 }
 
diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 259ec3bb8b6f..8db19d50c467 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -27,6 +27,8 @@
 
 #include <linux/init.h>
 #include <linux/module.h>
+#include <linux/list.h>
+#include <linux/hash.h>
 
 #include <asm/cache.h>
 #include <asm/setup.h>
@@ -37,6 +39,8 @@
 
 #include "xen-ops.h"
 
+static void __init m2p_override_init(void);
+
 unsigned long xen_max_p2m_pfn __read_mostly;
 
 #define P2M_PER_PAGE            (PAGE_SIZE / sizeof(unsigned long))
@@ -234,6 +238,8 @@ void __init xen_build_dynamic_phys_to_machine(void)
 
                 p2m_top[topidx][mididx] = &mfn_list[pfn];
         }
+
+        m2p_override_init();
 }
 
 unsigned long get_phys_to_machine(unsigned long pfn)
@@ -374,3 +380,77 @@ bool set_phys_to_machine(unsigned long pfn, unsigned long mfn)
 
         return true;
 }
+
+#define M2P_OVERRIDE_HASH_SHIFT 10
+#define M2P_OVERRIDE_HASH       (1 << M2P_OVERRIDE_HASH_SHIFT)
+
+static RESERVE_BRK_ARRAY(struct list_head, m2p_overrides, M2P_OVERRIDE_HASH);
+static DEFINE_SPINLOCK(m2p_override_lock);
+
+static void __init m2p_override_init(void)
+{
+        unsigned i;
+
+        m2p_overrides = extend_brk(sizeof(*m2p_overrides) * M2P_OVERRIDE_HASH,
+                                   sizeof(unsigned long));
+
+        for (i = 0; i < M2P_OVERRIDE_HASH; i++)
+                INIT_LIST_HEAD(&m2p_overrides[i]);
+}
+
+static unsigned long mfn_hash(unsigned long mfn)
+{
+        return hash_long(mfn, M2P_OVERRIDE_HASH_SHIFT);
+}
+
+/* Add an MFN override for a particular page */
+void m2p_add_override(unsigned long mfn, struct page *page)
+{
+        unsigned long flags;
+        page->private = mfn;
+
+        spin_lock_irqsave(&m2p_override_lock, flags);
+        list_add(&page->lru,  &m2p_overrides[mfn_hash(mfn)]);
+        spin_unlock_irqrestore(&m2p_override_lock, flags);
+}
+
+void m2p_remove_override(struct page *page)
+{
+        unsigned long flags;
+        spin_lock_irqsave(&m2p_override_lock, flags);
+        list_del(&page->lru);
+        spin_unlock_irqrestore(&m2p_override_lock, flags);
+}
+
+struct page *m2p_find_override(unsigned long mfn)
+{
+        unsigned long flags;
+        struct list_head *bucket = &m2p_overrides[mfn_hash(mfn)];
+        struct page *p, *ret;
+
+        ret = NULL;
+
+        spin_lock_irqsave(&m2p_override_lock, flags);
+
+        list_for_each_entry(p, bucket, lru) {
+                if (p->private == mfn) {
+                        ret = p;
+                        break;
+                }
+        }
+
+        spin_unlock_irqrestore(&m2p_override_lock, flags);
+
+        return ret;
+}
+
+unsigned long m2p_find_override_pfn(unsigned long mfn, unsigned long pfn)
+{
+        struct page *p = m2p_find_override(mfn);
+        unsigned long ret = pfn;
+
+        if (p)
+                ret = page_to_pfn(p);
+
+        return ret;
+}