[Xen-devel] [PATCH v4] altp2m: Allow the hostp2m entries to be of type p2m_ram_shared

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

Move sharing locks above altp2m to avoid locking order violation. Allow
applying altp2m mem_access settings when the hostp2m entries are shared.
Also, do not trigger PoD for hostp2m when setting altp2m mem_access to be
in-line with non-altp2m mem_access path. Also allow gfn remapping with
p2m_ram_shared type gfns in altp2m views.

When using mem_sharing in combination with altp2m, unsharing events overwrite
altp2m entries with that of the hostp2m (both remapped entries and mem_access
settings). User should take precaution to not share pages where this behavior
is undesired.

Signed-off-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
---
Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

v4: Resolve locking problem by moving sharing locks above altp2m locks
---
 xen/arch/x86/mm/mm-locks.h | 30 +++++++++++++++---------------
 xen/arch/x86/mm/p2m.c      | 10 +++++-----
 2 files changed, 20 insertions(+), 20 deletions(-)

diff --git a/xen/arch/x86/mm/mm-locks.h b/xen/arch/x86/mm/mm-locks.h
index 086c8bb..74fdfc1 100644
--- a/xen/arch/x86/mm/mm-locks.h
+++ b/xen/arch/x86/mm/mm-locks.h
@@ -242,6 +242,21 @@ declare_mm_lock(nestedp2m)
 
 declare_mm_rwlock(p2m);
 
+/* Sharing per page lock
+ *
+ * This is an external lock, not represented by an mm_lock_t. The memory
+ * sharing lock uses it to protect addition and removal of (gfn,domain)
+ * tuples to a shared page. We enforce order here against the p2m lock,
+ * which is taken after the page_lock to change the gfn's p2m entry.
+ *
+ * The lock is recursive because during share we lock two pages. */
+
+declare_mm_order_constraint(per_page_sharing)
+#define page_sharing_mm_pre_lock()   
mm_enforce_order_lock_pre_per_page_sharing()
+#define page_sharing_mm_post_lock(l, r) \
+        mm_enforce_order_lock_post_per_page_sharing((l), (r))
+#define page_sharing_mm_unlock(l, r) mm_enforce_order_unlock((l), (r))
+
 /* Alternate P2M list lock (per-domain)
  *
  * A per-domain lock that protects the list of alternate p2m's.
@@ -287,21 +302,6 @@ declare_mm_rwlock(altp2m);
 #define p2m_locked_by_me(p)   mm_write_locked_by_me(&(p)->lock)
 #define gfn_locked_by_me(p,g) p2m_locked_by_me(p)
 
-/* Sharing per page lock
- *
- * This is an external lock, not represented by an mm_lock_t. The memory
- * sharing lock uses it to protect addition and removal of (gfn,domain)
- * tuples to a shared page. We enforce order here against the p2m lock,
- * which is taken after the page_lock to change the gfn's p2m entry.
- *
- * The lock is recursive because during share we lock two pages. */
-
-declare_mm_order_constraint(per_page_sharing)
-#define page_sharing_mm_pre_lock()   
mm_enforce_order_lock_pre_per_page_sharing()
-#define page_sharing_mm_post_lock(l, r) \
-        mm_enforce_order_lock_post_per_page_sharing((l), (r))
-#define page_sharing_mm_unlock(l, r) mm_enforce_order_unlock((l), (r))
-
 /* PoD lock (per-p2m-table)
  * 
  * Protects private PoD data structs: entry and cache
diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index 9b19769..dc97082 100644
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -1768,10 +1768,10 @@ int p2m_set_altp2m_mem_access(struct domain *d, struct 
p2m_domain *hp2m,
     if ( !mfn_valid(mfn) )
     {
         mfn = hp2m->get_entry(hp2m, gfn_l, &t, &old_a,
-                              P2M_ALLOC | P2M_UNSHARE, &page_order, NULL);
+                              0, &page_order, NULL);
 
         rc = -ESRCH;
-        if ( !mfn_valid(mfn) || t != p2m_ram_rw )
+        if ( !mfn_valid(mfn) || (t != p2m_ram_rw && t != p2m_ram_shared) )
             return rc;
 
         /* If this is a superpage, copy that first */
@@ -2542,9 +2542,9 @@ int p2m_change_altp2m_gfn(struct domain *d, unsigned int 
idx,
     if ( !mfn_valid(mfn) )
     {
         mfn = hp2m->get_entry(hp2m, gfn_x(old_gfn), &t, &a,
-                              P2M_ALLOC | P2M_UNSHARE, &page_order, NULL);
+                              P2M_ALLOC, &page_order, NULL);
 
-        if ( !mfn_valid(mfn) || t != p2m_ram_rw )
+        if ( !mfn_valid(mfn) || (t != p2m_ram_rw && t != p2m_ram_shared) )
             goto out;
 
         /* If this is a superpage, copy that first */
@@ -2567,7 +2567,7 @@ int p2m_change_altp2m_gfn(struct domain *d, unsigned int 
idx,
     if ( !mfn_valid(mfn) )
         mfn = hp2m->get_entry(hp2m, gfn_x(new_gfn), &t, &a, 0, NULL, NULL);
 
-    if ( !mfn_valid(mfn) || (t != p2m_ram_rw) )
+    if ( !mfn_valid(mfn) || (t != p2m_ram_rw && t != p2m_ram_shared) )
         goto out;
 
     if ( !ap2m->set_entry(ap2m, gfn_x(old_gfn), mfn, PAGE_ORDER_4K, t, a,
-- 
2.8.1


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