[PATCH v1 5/6] xen/arm: io: Add a read-const writes-ignored mmio handler

["Edgar E. Iglesias" <edgar.iglesias@xxxxxxxxx>]

From: "Edgar E. Iglesias" <edgar.iglesias@xxxxxxx>

Add a read-const writes-ignored mmio handler. This is useful
to for example register background regions that return a fixed
value instead of raising data aborts.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xxxxxxx>
---
 xen/arch/arm/include/asm/mmio.h |  2 ++
 xen/arch/arm/io.c               | 21 +++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/xen/arch/arm/include/asm/mmio.h b/xen/arch/arm/include/asm/mmio.h
index 7da542cd79..605620a2f4 100644
--- a/xen/arch/arm/include/asm/mmio.h
+++ b/xen/arch/arm/include/asm/mmio.h
@@ -82,6 +82,8 @@ struct vmmio {
     struct mmio_handler *handlers;
 };
 
+extern const struct mmio_handler_ops mmio_read_const_writes_ignored;
+
 enum io_state try_handle_mmio(struct cpu_user_regs *regs,
                               mmio_info_t *info);
 void register_mmio_bg_handler(struct domain *d,
diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
index 934a2ad2b9..8ab0435afc 100644
--- a/xen/arch/arm/io.c
+++ b/xen/arch/arm/io.c
@@ -20,6 +20,27 @@
 
 #include "decode.h"
 
+/*
+ * Reusable mmio handler useful as background while waiting for IOREQ.
+ * Register with priv as default read value. Writes ignored.
+ */
+static int bg_read(struct vcpu *v, mmio_info_t *info, register_t *r, void 
*priv)
+{
+    *r = (uintptr_t) priv;
+    return 1;
+}
+
+static int bg_write(struct vcpu *v, mmio_info_t *info, register_t r, void 
*priv)
+{
+    return 1;
+}
+
+/* Read const value (from priv), writes ignored.  */
+const struct mmio_handler_ops mmio_read_const_writes_ignored = {
+    .read = bg_read,
+    .write = bg_write,
+};
+
 static enum io_state handle_read(const struct mmio_handler *handler,
                                  struct vcpu *v,
                                  mmio_info_t *info)
-- 
2.43.0