[PATCH 3 of 3] x86/emulation: No need to get_gfn on zero ram_gpa

[Andres Lagar-Cavilla <andres@lagarcavilla.org>]

 xen/arch/x86/hvm/emulate.c |  48 ++++++++++++++++++++++++---------------------
 1 files changed, 26 insertions(+), 22 deletions(-)


Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>

diff -r 2ffc676120b8 -r 7a7443e80b99 xen/arch/x86/hvm/emulate.c
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -60,33 +60,37 @@ static int hvmemul_do_io(
     ioreq_t *p = get_ioreq(curr);
     unsigned long ram_gfn = paddr_to_pfn(ram_gpa);
     p2m_type_t p2mt;
-    mfn_t ram_mfn;
+    mfn_t ram_mfn = _mfn(INVALID_MFN);
     int rc;
 
-    /* Check for paged out page */
-    ram_mfn = get_gfn_unshare(curr->domain, ram_gfn, &p2mt);
-    if ( p2m_is_paging(p2mt) )
-    {
-        put_gfn(curr->domain, ram_gfn); 
-        p2m_mem_paging_populate(curr->domain, ram_gfn);
-        return X86EMUL_RETRY;
-    }
-    if ( p2m_is_shared(p2mt) )
-    {
-        put_gfn(curr->domain, ram_gfn); 
-        return X86EMUL_RETRY;
-    }
-
-    /* Maintain a ref on the mfn to ensure liveness. Put the gfn
-     * to avoid potential deadlock wrt event channel lock, later. */
-    if ( mfn_valid(mfn_x(ram_mfn)) )
-        if ( !get_page(mfn_to_page(mfn_x(ram_mfn)),
-             curr->domain) )
+    /* Many callers pass a stub zero ram_gpa address. */
+    if ( ram_gfn != 0 )
+    { 
+        /* Check for paged out page */
+        ram_mfn = get_gfn_unshare(curr->domain, ram_gfn, &p2mt);
+        if ( p2m_is_paging(p2mt) )
         {
-            put_gfn(curr->domain, ram_gfn);
+            put_gfn(curr->domain, ram_gfn); 
+            p2m_mem_paging_populate(curr->domain, ram_gfn);
             return X86EMUL_RETRY;
         }
-    put_gfn(curr->domain, ram_gfn);
+        if ( p2m_is_shared(p2mt) )
+        {
+            put_gfn(curr->domain, ram_gfn); 
+            return X86EMUL_RETRY;
+        }
+
+        /* Maintain a ref on the mfn to ensure liveness. Put the gfn
+         * to avoid potential deadlock wrt event channel lock, later. */
+        if ( mfn_valid(mfn_x(ram_mfn)) )
+            if ( !get_page(mfn_to_page(mfn_x(ram_mfn)),
+                 curr->domain) )
+            {
+                put_gfn(curr->domain, ram_gfn);
+                return X86EMUL_RETRY;
+            }
+        put_gfn(curr->domain, ram_gfn);
+    }
 
     /*
      * Weird-sized accesses have undefined behaviour: we discard writes