dataplane: add host memory mapping code

The data plane thread needs to map guest physical addresses to host
pointers.  Normally this is done with cpu_physical_memory_map() but the
function assumes the global mutex is held.  The data plane thread does
not touch the global mutex and therefore needs a thread-safe memory
mapping mechanism.

Hostmem registers a MemoryListener similar to how vhost collects and
pushes memory region information into the kernel.  There is a
fine-grained lock on the regions list which is held during lookup and
when installing a new regions list.

When the physical memory map changes the MemoryListener callbacks are
invoked.  They build up a new list of memory regions which is finally
installed when the list has been completed.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

hw/dataplane/hostmem.h

@@ -0,0 +1,57 @@
+/*
+ * Thread-safe guest to host memory mapping
+ *
+ * Copyright 2012 Red Hat, Inc. and/or its affiliates
+ *
+ * Authors:
+ *   Stefan Hajnoczi <stefanha@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+
+#ifndef HOSTMEM_H
+#define HOSTMEM_H
+
+#include "exec/memory.h"
+#include "qemu/thread.h"
+
+typedef struct {
+    void *host_addr;
+    hwaddr guest_addr;
+    uint64_t size;
+    bool readonly;
+} HostMemRegion;
+
+typedef struct {
+    /* The listener is invoked when regions change and a new list of regions is
+     * built up completely before they are installed.
+     */
+    MemoryListener listener;
+    HostMemRegion *new_regions;
+    size_t num_new_regions;
+
+    /* Current regions are accessed from multiple threads either to lookup
+     * addresses or to install a new list of regions.  The lock protects the
+     * pointer and the regions.
+     */
+    QemuMutex current_regions_lock;
+    HostMemRegion *current_regions;
+    size_t num_current_regions;
+} HostMem;
+
+void hostmem_init(HostMem *hostmem);
+void hostmem_finalize(HostMem *hostmem);
+
+/**
+ * Map a guest physical address to a pointer
+ *
+ * Note that there is map/unmap mechanism here.  The caller must ensure that
+ * mapped memory is no longer used across events like hot memory unplug.  This
+ * can be done with other mechanisms like bdrv_drain_all() that quiesce
+ * in-flight I/O.
+ */
+void *hostmem_lookup(HostMem *hostmem, hwaddr phys, hwaddr len, bool is_write);
+
+#endif /* HOSTMEM_H */