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>

diff --git a/arch/x86/xen/multicalls.c b/arch/x86/xen/multicalls.c
index cc71f7c..2474553 100644 (file)
--- 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];