xen: add kconfig for event_fifo

Evtchn fifos are not needed on smaller systems; the older interface is
lightweight and sufficient. Also, event_fifo causes runtime anonymous
memory allocations, which are undesirable.  Additionally, it exposes an
extra interface to the guest, which is also undesirable unless
necessary.

Make it possible to disable evtchn fifo.

Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Acked-by: Michal Orzel <michal.orzel@amd.com>

xen/common/Kconfig

@@ -23,6 +23,20 @@ config GRANT_TABLE
 
 	  If unsure, say Y.
 
+config EVTCHN_FIFO
+	bool "Event Channel Fifo support" if EXPERT
+	default y
+	help
+	  The Event channel Fifo extends support for event channels
+	  beyond 1024 event channels for 32-bit guests and 4096 for
+	  64-bit guests.
+
+	  Cons: They cause runtime anonymous memory allocations and expose
+	  an additional interface to the guest. For smaller systems, you
+	  might consider disabling this feature.
+
+	  If unsure, say Y.
+
 config PDX_COMPRESSION
 	bool "PDX (Page inDeX) compression" if EXPERT && !X86 && !RISCV
 	default ARM || PPC

xen/common/Makefile

@@ -13,7 +13,7 @@ obj-$(CONFIG_IOREQ_SERVER) += dm.o
 obj-y += domain.o
 obj-y += event_2l.o
 obj-y += event_channel.o
-obj-y += event_fifo.o
+obj-$(CONFIG_EVTCHN_FIFO) += event_fifo.o
 obj-$(CONFIG_GRANT_TABLE) += grant_table.o
 obj-y += guestcopy.o
 obj-y += gzip/

xen/common/event_channel.h

@@ -45,12 +45,27 @@ void evtchn_2l_init(struct domain *d);
 
 /* FIFO */
 
+#ifdef CONFIG_EVTCHN_FIFO
 struct evtchn_init_control;
 struct evtchn_expand_array;
 
 int evtchn_fifo_init_control(struct evtchn_init_control *init_control);
 int evtchn_fifo_expand_array(const struct evtchn_expand_array *expand_array);
 void evtchn_fifo_destroy(struct domain *d);
+#else
+static inline int evtchn_fifo_init_control(struct evtchn_init_control *init_control)
+{
+    return -EOPNOTSUPP;
+}
+static inline int evtchn_fifo_expand_array(const struct evtchn_expand_array *expand_array)
+{
+    return -EOPNOTSUPP;
+}
+static inline void evtchn_fifo_destroy(struct domain *d)
+{
+    return;
+}
+#endif /* CONFIG_EVTCHN_FIFO */
 
 /*
  * Local variables: