memory: Introduce memory_region_init_reservation

Introduce a memory region type that can reserve I/O space. Such regions
are useful for modeling I/O that is only handled outside of QEMU, i.e.
in the context of an accelerator like KVM.

Any access to such a region from QEMU is a bug, but could theoretically
be triggered by guest code (DMA to reserved region). So only warning
about such events once, then ignore them.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

memory.c

@@ -1049,6 +1049,42 @@ void memory_region_init_rom_device(MemoryRegion *mr,
     mr->ram_addr |= cpu_register_io_memory(mr);
 }
 
+static uint64_t invalid_read(void *opaque, target_phys_addr_t addr,
+                             unsigned size)
+{
+    MemoryRegion *mr = opaque;
+
+    if (!mr->warning_printed) {
+        fprintf(stderr, "Invalid read from memory region %s\n", mr->name);
+        mr->warning_printed = true;
+    }
+    return -1U;
+}
+
+static void invalid_write(void *opaque, target_phys_addr_t addr, uint64_t data,
+                          unsigned size)
+{
+    MemoryRegion *mr = opaque;
+
+    if (!mr->warning_printed) {
+        fprintf(stderr, "Invalid write to memory region %s\n", mr->name);
+        mr->warning_printed = true;
+    }
+}
+
+static const MemoryRegionOps reservation_ops = {
+    .read = invalid_read,
+    .write = invalid_write,
+    .endianness = DEVICE_NATIVE_ENDIAN,
+};
+
+void memory_region_init_reservation(MemoryRegion *mr,
+                                    const char *name,
+                                    uint64_t size)
+{
+    memory_region_init_io(mr, &reservation_ops, mr, name, size);
+}
+
 void memory_region_destroy(MemoryRegion *mr)
 {
     assert(QTAILQ_EMPTY(&mr->subregions));

memory.h

@@ -126,6 +126,7 @@ struct MemoryRegion {
     bool readonly; /* For RAM regions */
     bool enabled;
     bool rom_device;
+    bool warning_printed; /* For reservations */
     MemoryRegion *alias;
     target_phys_addr_t alias_offset;
     unsigned priority;
@@ -279,6 +280,21 @@ void memory_region_init_rom_device(MemoryRegion *mr,
                                    const char *name,
                                    uint64_t size);
 
+/**
+ * memory_region_init_reservation: Initialize a memory region that reserves
+ *                                 I/O space.
+ *
+ * A reservation region primariy serves debugging purposes.  It claims I/O
+ * space that is not supposed to be handled by QEMU itself.  Any access via
+ * the memory API will cause an abort().
+ *
+ * @mr: the #MemoryRegion to be initialized
+ * @name: used for debugging; not visible to the user or ABI
+ * @size: size of the region.
+ */
+void memory_region_init_reservation(MemoryRegion *mr,
+                                    const char *name,
+                                    uint64_t size);
 /**
  * memory_region_destroy: Destroy a memory region and reclaim all resources.
  *