hw/xen: Add xenstore wire implementation and implementation stubs

This implements the basic wire protocol for the XenStore commands, punting
all the actual implementation to xs_impl_* functions which all just return
errors for now.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>

hw/i386/kvm/xenstore_impl.c

@@ -0,0 +1,117 @@
+/*
+ * QEMU Xen emulation: The actual implementation of XenStore
+ *
+ * Copyright © 2023 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Authors: David Woodhouse <dwmw2@infradead.org>, Paul Durrant <paul@xen.org>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#include "qemu/osdep.h"
+
+#include "xen_xenstore.h"
+#include "xenstore_impl.h"
+
+struct XenstoreImplState {
+};
+
+int xs_impl_read(XenstoreImplState *s, unsigned int dom_id,
+                 xs_transaction_t tx_id, const char *path, GByteArray *data)
+{
+    /*
+     * The data GByteArray shall exist, and will be freed by caller.
+     * Just g_byte_array_append() to it.
+     */
+    return ENOENT;
+}
+
+int xs_impl_write(XenstoreImplState *s, unsigned int dom_id,
+                  xs_transaction_t tx_id, const char *path, GByteArray *data)
+{
+    /*
+     * The data GByteArray shall exist, will be freed by caller. You are
+     * free to use g_byte_array_steal() and keep the data.
+     */
+    return ENOSYS;
+}
+
+int xs_impl_directory(XenstoreImplState *s, unsigned int dom_id,
+                      xs_transaction_t tx_id, const char *path,
+                      uint64_t *gencnt, GList **items)
+{
+    /*
+     * The items are (char *) to be freed by caller. Although it's consumed
+     * immediately so if you want to change it to (const char *) and keep
+     * them, go ahead and change the caller.
+     */
+    return ENOENT;
+}
+
+int xs_impl_transaction_start(XenstoreImplState *s, unsigned int dom_id,
+                              xs_transaction_t *tx_id)
+{
+    return ENOSYS;
+}
+
+int xs_impl_transaction_end(XenstoreImplState *s, unsigned int dom_id,
+                            xs_transaction_t tx_id, bool commit)
+{
+    return ENOSYS;
+}
+
+int xs_impl_rm(XenstoreImplState *s, unsigned int dom_id,
+               xs_transaction_t tx_id, const char *path)
+{
+    return ENOSYS;
+}
+
+int xs_impl_get_perms(XenstoreImplState *s, unsigned int dom_id,
+                      xs_transaction_t tx_id, const char *path, GList **perms)
+{
+    /*
+     * The perms are (char *) in the <perm-as-string> wire format to be
+     * freed by the caller.
+     */
+    return ENOSYS;
+}
+
+int xs_impl_set_perms(XenstoreImplState *s, unsigned int dom_id,
+                      xs_transaction_t tx_id, const char *path, GList *perms)
+{
+    /*
+     * The perms are (const char *) in the <perm-as-string> wire format.
+     */
+    return ENOSYS;
+}
+
+int xs_impl_watch(XenstoreImplState *s, unsigned int dom_id, const char *path,
+                  const char *token, xs_impl_watch_fn fn, void *opaque)
+{
+    /*
+     * When calling the callback @fn, note that the path should
+     * precisely match the relative path that the guest provided, even
+     * if it was a relative path which needed to be prefixed with
+     * /local/domain/${domid}/
+     */
+    return ENOSYS;
+}
+
+int xs_impl_unwatch(XenstoreImplState *s, unsigned int dom_id,
+                    const char *path, const char *token,
+                    xs_impl_watch_fn fn, void *opaque)
+{
+    /* Remove the watch that matches all four criteria */
+    return ENOSYS;
+}
+
+int xs_impl_reset_watches(XenstoreImplState *s, unsigned int dom_id)
+{
+    return ENOSYS;
+}
+
+XenstoreImplState *xs_impl_create(void)
+{
+    return g_new0(XenstoreImplState, 1);
+}