xen/multicall: special-case singleton hypercalls

Singleton calls seem to end up being pretty common, so just
directly call the hypercall rather than going via multicall.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

1 files changed, 22 insertions, 5 deletions

diff --git a/arch/x86/xen/multicalls.c b/arch/x86/xen/multicalls.c
index cc71f7cf1adc..2474553d1f26 100644
--- a/arch/x86/xen/multicalls.c
+++ b/arch/x86/xen/multicalls.c
@@ -55,6 +55,7 @@ DEFINE_PER_CPU(unsigned long, xen_mc_irq_flags);
 void xen_mc_flush(void)
 {
         struct mc_buffer *b = &__get_cpu_var(mc_buffer);
+        struct multicall_entry *mc;
         int ret = 0;
         unsigned long flags;
         int i;
@@ -67,7 +68,24 @@ void xen_mc_flush(void)
 
         trace_xen_mc_flush(b->mcidx, b->argidx, b->cbidx);
 
-        if (b->mcidx) {
+        switch (b->mcidx) {
+        case 0:
+                /* no-op */
+                BUG_ON(b->argidx != 0);
+                break;
+
+        case 1:
+                /* Singleton multicall - bypass multicall machinery
+                   and just do the call directly. */
+                mc = &b->entries[0];
+
+                mc->result = privcmd_call(mc->op,
+                                          mc->args[0], mc->args[1], mc->args[2], 
+                                          mc->args[3], mc->args[4]);
+                ret = mc->result < 0;
+                break;
+
+        default:
 #if MC_DEBUG
                 memcpy(b->debug, b->entries,
                        b->mcidx * sizeof(struct multicall_entry));
@@ -94,11 +112,10 @@ void xen_mc_flush(void)
                         }
                 }
 #endif
+        }
 
-                b->mcidx = 0;
-                b->argidx = 0;
-        } else
-                BUG_ON(b->argidx != 0);
+        b->mcidx = 0;
+        b->argidx = 0;
 
         for (i = 0; i < b->cbidx; i++) {
                 struct callback *cb = &b->callbacks[i];