hw/xen: Implement XenStore watches
Starts out fairly simple: a hash table of watches based on the path.
Except there can be multiple watches on the same path, so the watch ends
up being a simple linked list, and the head of that list is in the hash
table. Which makes removal a bit of a PITA but it's not so bad; we just
special-case "I had to remove the head of the list and now I have to
replace it in / remove it from the hash table". And if we don't remove
the head, it's a simple linked-list operation.
We do need to fire watches on *deleted* nodes, so instead of just a simple
xs_node_unref() on the topmost victim, we need to recurse down and fire
watches on them all.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
diff --git a/hw/i386/kvm/xenstore_impl.c b/hw/i386/kvm/xenstore_impl.c
index 9e10a31..9c23488 100644
--- a/hw/i386/kvm/xenstore_impl.c
+++ b/hw/i386/kvm/xenstore_impl.c
@@ -37,9 +37,20 @@
#endif
} XsNode;
+typedef struct XsWatch {
+ struct XsWatch *next;
+ xs_impl_watch_fn *cb;
+ void *cb_opaque;
+ char *token;
+ unsigned int dom_id;
+ int rel_prefix;
+} XsWatch;
+
struct XenstoreImplState {
XsNode *root;
unsigned int nr_nodes;
+ GHashTable *watches;
+ unsigned int nr_domu_watches;
};
static inline XsNode *xs_node_new(void)
@@ -146,6 +157,7 @@
void *op_opaque;
void *op_opaque2;
+ GList *watches;
unsigned int dom_id;
/* The number of nodes which will exist in the tree if this op succeeds. */
@@ -166,6 +178,35 @@
bool create_dirs;
};
+static void fire_watches(struct walk_op *op, bool parents)
+{
+ GList *l = NULL;
+ XsWatch *w;
+
+ if (!op->mutating) {
+ return;
+ }
+
+ if (parents) {
+ l = op->watches;
+ }
+
+ w = g_hash_table_lookup(op->s->watches, op->path);
+ while (w || l) {
+ if (!w) {
+ /* Fire the parent nodes from 'op' if asked to */
+ w = l->data;
+ l = l->next;
+ continue;
+ }
+
+ assert(strlen(op->path) > w->rel_prefix);
+ w->cb(w->cb_opaque, op->path + w->rel_prefix, w->token);
+
+ w = w->next;
+ }
+}
+
static int xs_node_add_content(XsNode **n, struct walk_op *op)
{
GByteArray *data = op->op_opaque;
@@ -213,6 +254,8 @@
static int node_rm_recurse(gpointer key, gpointer value, gpointer user_data)
{
struct walk_op *op = user_data;
+ int path_len = strlen(op->path);
+ int key_len = strlen(key);
XsNode *n = value;
bool this_inplace = op->inplace;
@@ -220,12 +263,23 @@
op->inplace = 0;
}
+ assert(key_len + path_len + 2 <= sizeof(op->path));
+ op->path[path_len] = '/';
+ memcpy(op->path + path_len + 1, key, key_len + 1);
+
if (n->children) {
g_hash_table_foreach_remove(n->children, node_rm_recurse, op);
}
op->new_nr_nodes--;
/*
+ * Fire watches on *this* node but not the parents because they are
+ * going to be deleted too, so the watch will fire for them anyway.
+ */
+ fire_watches(op, false);
+ op->path[path_len] = '\0';
+
+ /*
* Actually deleting the child here is just an optimisation; if we
* don't then the final unref on the topmost victim will just have
* to cascade down again repeating all the g_hash_table_foreach()
@@ -238,7 +292,7 @@
{
bool this_inplace = op->inplace;
- /* Keep count of the nodes in the subtree which gets deleted. */
+ /* Fire watches for, and count, nodes in the subtree which get deleted */
if ((*n)->children) {
g_hash_table_foreach_remove((*n)->children, node_rm_recurse, op);
}
@@ -269,9 +323,11 @@
XsNode *old = *n, *child = NULL;
bool stole_child = false;
bool this_inplace;
+ XsWatch *watch;
int err;
namelen = strlen(op->path);
+ watch = g_hash_table_lookup(op->s->watches, op->path);
/* Is there a child, or do we hit the double-NUL termination? */
if (op->path[namelen + 1]) {
@@ -292,6 +348,9 @@
if (!child_name) {
/* This is the actual node on which the operation shall be performed */
err = op->op_fn(n, op);
+ if (!err) {
+ fire_watches(op, true);
+ }
goto out;
}
@@ -334,10 +393,23 @@
}
/*
+ * If there's a watch on this node, add it to the list to be fired
+ * (with the correct full pathname for the modified node) at the end.
+ */
+ if (watch) {
+ op->watches = g_list_append(op->watches, watch);
+ }
+
+ /*
* Except for the temporary child-stealing as noted, our node has not
* changed yet. We don't yet know the overall operation will complete.
*/
err = xs_node_walk(&child, op);
+
+ if (watch) {
+ op->watches = g_list_remove(op->watches, watch);
+ }
+
if (err || !op->mutating) {
if (stole_child) {
/* Put it back as it was. */
@@ -375,6 +447,7 @@
out:
op->path[namelen] = '\0';
if (!namelen) {
+ assert(!op->watches);
/*
* On completing the recursion back up the path walk and reaching the
* top, assign the new node count if the operation was successful.
@@ -457,6 +530,7 @@
* path element for the lookup.
*/
op->path[strlen(op->path) + 1] = '\0';
+ op->watches = NULL;
op->path[0] = '\0';
op->inplace = true;
op->mutating = false;
@@ -589,38 +663,187 @@
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/${domid}/
- */
- return ENOSYS;
+ char abspath[XENSTORE_ABS_PATH_MAX + 1];
+ XsWatch *w, *l;
+ int ret;
+
+ ret = validate_path(abspath, path, dom_id);
+ if (ret) {
+ return ret;
+ }
+
+ /* Check for duplicates */
+ l = w = g_hash_table_lookup(s->watches, abspath);
+ while (w) {
+ if (!g_strcmp0(token, w->token) && opaque == w->cb_opaque &&
+ fn == w->cb && dom_id == w->dom_id) {
+ return EEXIST;
+ }
+ w = w->next;
+ }
+
+ if (dom_id && s->nr_domu_watches >= XS_MAX_WATCHES) {
+ return E2BIG;
+ }
+
+ w = g_new0(XsWatch, 1);
+ w->token = g_strdup(token);
+ w->cb = fn;
+ w->cb_opaque = opaque;
+ w->dom_id = dom_id;
+ w->rel_prefix = strlen(abspath) - strlen(path);
+
+ /* l was looked up above when checking for duplicates */
+ if (l) {
+ w->next = l->next;
+ l->next = w;
+ } else {
+ g_hash_table_insert(s->watches, g_strdup(abspath), w);
+ }
+ if (dom_id) {
+ s->nr_domu_watches++;
+ }
+
+ /* A new watch should fire immediately */
+ fn(opaque, path, token);
+
+ return 0;
+}
+
+static XsWatch *free_watch(XenstoreImplState *s, XsWatch *w)
+{
+ XsWatch *next = w->next;
+
+ if (w->dom_id) {
+ assert(s->nr_domu_watches);
+ s->nr_domu_watches--;
+ }
+
+ g_free(w->token);
+ g_free(w);
+
+ return next;
}
int xs_impl_unwatch(XenstoreImplState *s, unsigned int dom_id,
const char *path, const char *token,
xs_impl_watch_fn fn, void *opaque)
{
+ char abspath[XENSTORE_ABS_PATH_MAX + 1];
+ XsWatch *w, **l;
+ int ret;
+
+ ret = validate_path(abspath, path, dom_id);
+ if (ret) {
+ return ret;
+ }
+
+ w = g_hash_table_lookup(s->watches, abspath);
+ if (!w) {
+ return ENOENT;
+ }
+
/*
- * When calling the callback @fn, note that the path should
- * precisely match the relative path that the guest provided, even
- * if it was a relative path which needed to be prefixed with
- * /local/domain/${domid}/
+ * The hash table contains the first element of a list of
+ * watches. Removing the first element in the list is a
+ * special case because we have to update the hash table to
+ * point to the next (or remove it if there's nothing left).
*/
- return ENOSYS;
+ if (!g_strcmp0(token, w->token) && fn == w->cb && opaque == w->cb_opaque &&
+ dom_id == w->dom_id) {
+ if (w->next) {
+ /* Insert the previous 'next' into the hash table */
+ g_hash_table_insert(s->watches, g_strdup(abspath), w->next);
+ } else {
+ /* Nothing left; remove from hash table */
+ g_hash_table_remove(s->watches, abspath);
+ }
+ free_watch(s, w);
+ return 0;
+ }
+
+ /*
+ * We're all done messing with the hash table because the element
+ * it points to has survived the cull. Now it's just a simple
+ * linked list removal operation.
+ */
+ for (l = &w->next; *l; l = &w->next) {
+ w = *l;
+
+ if (!g_strcmp0(token, w->token) && fn == w->cb &&
+ opaque != w->cb_opaque && dom_id == w->dom_id) {
+ *l = free_watch(s, w);
+ return 0;
+ }
+ }
+
+ return ENOENT;
}
int xs_impl_reset_watches(XenstoreImplState *s, unsigned int dom_id)
{
- /* Remove the watch that matches all four criteria */
- return ENOSYS;
+ char **watch_paths;
+ guint nr_watch_paths;
+ guint i;
+
+ watch_paths = (char **)g_hash_table_get_keys_as_array(s->watches,
+ &nr_watch_paths);
+
+ for (i = 0; i < nr_watch_paths; i++) {
+ XsWatch *w1 = g_hash_table_lookup(s->watches, watch_paths[i]);
+ XsWatch *w2, *w, **l;
+
+ /*
+ * w1 is the original list. The hash table has this pointer.
+ * w2 is the head of our newly-filtered list.
+ * w and l are temporary for processing. w is somewhat redundant
+ * with *l but makes my eyes bleed less.
+ */
+
+ w = w2 = w1;
+ l = &w;
+ while (w) {
+ if (w->dom_id == dom_id) {
+ /* If we're freeing the head of the list, bump w2 */
+ if (w2 == w) {
+ w2 = w->next;
+ }
+ *l = free_watch(s, w);
+ } else {
+ l = &w->next;
+ }
+ w = *l;
+ }
+ /*
+ * If the head of the list survived the cull, we don't need to
+ * touch the hash table and we're done with this path. Else...
+ */
+ if (w1 != w2) {
+ g_hash_table_steal(s->watches, watch_paths[i]);
+
+ /*
+ * It was already freed. (Don't worry, this whole thing is
+ * single-threaded and nobody saw it in the meantime). And
+ * having *stolen* it, we now own the watch_paths[i] string
+ * so if we don't give it back to the hash table, we need
+ * to free it.
+ */
+ if (w2) {
+ g_hash_table_insert(s->watches, watch_paths[i], w2);
+ } else {
+ g_free(watch_paths[i]);
+ }
+ }
+ }
+ g_free(watch_paths);
+ return 0;
}
XenstoreImplState *xs_impl_create(void)
{
XenstoreImplState *s = g_new0(XenstoreImplState, 1);
+ s->watches = g_hash_table_new_full(g_str_hash, g_str_equal, g_free, NULL);
s->nr_nodes = 1;
s->root = xs_node_new();
#ifdef XS_NODE_UNIT_TEST
diff --git a/tests/unit/test-xs-node.c b/tests/unit/test-xs-node.c
index c78d5bd..19000b6 100644
--- a/tests/unit/test-xs-node.c
+++ b/tests/unit/test-xs-node.c
@@ -34,10 +34,14 @@
{
int err;
+ xs_impl_reset_watches(s, DOMID_GUEST);
+ g_assert(!s->nr_domu_watches);
+
err = xs_impl_rm(s, DOMID_QEMU, XBT_NULL, "/local");
g_assert(!err);
g_assert(s->nr_nodes == 1);
+ g_hash_table_unref(s->watches);
xs_node_unref(s->root);
g_free(s);
@@ -65,6 +69,14 @@
return err;
}
+static void watch_cb(void *_str, const char *path, const char *token)
+{
+ GString *str = _str;
+
+ g_string_append(str, path);
+ g_string_append(str, token);
+}
+
static XenstoreImplState *setup(void)
{
XenstoreImplState *s = xs_impl_create();
@@ -75,6 +87,7 @@
err = write_str(s, DOMID_QEMU, XBT_NULL, abspath, "");
g_assert(!err);
+ g_assert(s->nr_nodes == 4);
g_free(abspath);
@@ -93,6 +106,8 @@
{
GByteArray *data = g_byte_array_new();
XenstoreImplState *s = setup();
+ GString *guest_watches = g_string_new(NULL);
+ GString *qemu_watches = g_string_new(NULL);
GList *items = NULL;
XsNode *old_root;
uint64_t gencnt;
@@ -100,6 +115,20 @@
g_assert(s);
+ err = xs_impl_watch(s, DOMID_GUEST, "some", "guestwatch",
+ watch_cb, guest_watches);
+ g_assert(!err);
+ g_assert(guest_watches->len == strlen("someguestwatch"));
+ g_assert(!strcmp(guest_watches->str, "someguestwatch"));
+ g_string_truncate(guest_watches, 0);
+
+ err = xs_impl_watch(s, 0, "/local/domain/1/some", "qemuwatch",
+ watch_cb, qemu_watches);
+ g_assert(!err);
+ g_assert(qemu_watches->len == strlen("/local/domain/1/someqemuwatch"));
+ g_assert(!strcmp(qemu_watches->str, "/local/domain/1/someqemuwatch"));
+ g_string_truncate(qemu_watches, 0);
+
/* Read gives ENOENT when it should */
err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "foo", data);
g_assert(err == ENOENT);
@@ -109,6 +138,14 @@
"something");
g_assert(s->nr_nodes == 7);
g_assert(!err);
+ g_assert(!strcmp(guest_watches->str,
+ "some/relative/pathguestwatch"));
+ g_assert(!strcmp(qemu_watches->str,
+ "/local/domain/1/some/relative/pathqemuwatch"));
+
+ g_string_truncate(qemu_watches, 0);
+ g_string_truncate(guest_watches, 0);
+ xs_impl_reset_watches(s, 0);
/* Read gives back what we wrote */
err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "some/relative/path", data);
@@ -123,6 +160,8 @@
g_assert(!err);
g_assert(data->len == strlen("something"));
+ g_assert(!qemu_watches->len);
+ g_assert(!guest_watches->len);
/* Keep a copy, to force COW mode */
old_root = xs_node_ref(s->root);
@@ -132,12 +171,18 @@
"something else");
g_assert(!err);
g_assert(s->nr_nodes == 8);
+ g_assert(!qemu_watches->len);
+ g_assert(!strcmp(guest_watches->str, "some/relative/path2guestwatch"));
+ g_string_truncate(guest_watches, 0);
/* Overwrite an existing node */
err = write_str(s, DOMID_GUEST, XBT_NULL, "some/relative/path",
"another thing");
g_assert(!err);
g_assert(s->nr_nodes == 8);
+ g_assert(!qemu_watches->len);
+ g_assert(!strcmp(guest_watches->str, "some/relative/pathguestwatch"));
+ g_string_truncate(guest_watches, 0);
/* We can list the two files we wrote */
err = xs_impl_directory(s, DOMID_GUEST, XBT_NULL, "some/relative", &gencnt,
@@ -151,9 +196,38 @@
g_assert(!items->next->next);
g_list_free_full(items, g_free);
+ err = xs_impl_unwatch(s, DOMID_GUEST, "some", "guestwatch",
+ watch_cb, guest_watches);
+ g_assert(!err);
+
+ err = xs_impl_unwatch(s, DOMID_GUEST, "some", "guestwatch",
+ watch_cb, guest_watches);
+ g_assert(err == ENOENT);
+
+ err = xs_impl_watch(s, DOMID_GUEST, "some/relative/path2", "watchp2",
+ watch_cb, guest_watches);
+ g_assert(!err);
+ g_assert(guest_watches->len == strlen("some/relative/path2watchp2"));
+ g_assert(!strcmp(guest_watches->str, "some/relative/path2watchp2"));
+ g_string_truncate(guest_watches, 0);
+
+ err = xs_impl_watch(s, DOMID_GUEST, "/local/domain/1/some/relative",
+ "watchrel", watch_cb, guest_watches);
+ g_assert(!err);
+ g_assert(guest_watches->len ==
+ strlen("/local/domain/1/some/relativewatchrel"));
+ g_assert(!strcmp(guest_watches->str,
+ "/local/domain/1/some/relativewatchrel"));
+ g_string_truncate(guest_watches, 0);
+
/* Write somewhere else which already existed */
err = write_str(s, DOMID_GUEST, XBT_NULL, "some/relative", "moredata");
g_assert(!err);
+ g_assert(s->nr_nodes == 8);
+
+ g_assert(!strcmp(guest_watches->str,
+ "/local/domain/1/some/relativewatchrel"));
+ g_string_truncate(guest_watches, 0);
g_byte_array_set_size(data, 0);
err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "some/relative", data);
@@ -164,6 +238,7 @@
/* Overwrite existing data */
err = write_str(s, DOMID_GUEST, XBT_NULL, "some/relative", "otherdata");
g_assert(!err);
+ g_string_truncate(guest_watches, 0);
g_byte_array_set_size(data, 0);
err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "some/relative", data);
@@ -176,11 +251,21 @@
g_assert(!err);
g_assert(s->nr_nodes == 5);
+ /* Each watch fires with the least specific relevant path */
+ g_assert(strstr(guest_watches->str,
+ "some/relative/path2watchp2"));
+ g_assert(strstr(guest_watches->str,
+ "/local/domain/1/some/relativewatchrel"));
+ g_string_truncate(guest_watches, 0);
+
g_byte_array_set_size(data, 0);
err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "some/relative", data);
g_assert(err == ENOENT);
g_byte_array_unref(data);
+ xs_impl_reset_watches(s, DOMID_GUEST);
+ g_string_free(qemu_watches, true);
+ g_string_free(guest_watches, true);
xs_node_unref(old_root);
xs_impl_delete(s);
}
