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hw/xen: Implement XenStore watches

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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>
This commit is contained in:
David Woodhouse 2023-01-22 18:38:23 +00:00
parent 3ef7ff83ca
commit 6e1330090d
2 changed files with 323 additions and 15 deletions
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253
hw/i386/kvm/xenstore_impl.c
View File

@ -37,9 +37,20 @@ typedef struct XsNode {
#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 @@ struct walk_op {
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 @@ struct walk_op {
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 xs_node_get_content(XsNode **n, struct walk_op *op)
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;
@ -220,11 +263,22 @@ static int node_rm_recurse(gpointer key, gpointer value, gpointer user_data)
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
@ -238,7 +292,7 @@ static int xs_node_rm(XsNode **n, struct walk_op *op)
{
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 @@ static int xs_node_walk(XsNode **n, struct walk_op *op)
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 @@ static int xs_node_walk(XsNode **n, struct walk_op *op)
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;
}
@ -333,11 +392,24 @@ static int xs_node_walk(XsNode **n, struct walk_op *op)
goto out;
}
/*
* 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 @@ static int xs_node_walk(XsNode **n, struct walk_op *op)
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 @@ static int init_walk_op(XenstoreImplState *s, struct walk_op *op,
* 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_set_perms(XenstoreImplState *s, unsigned int dom_id,
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

85
tests/unit/test-xs-node.c
View File

@ -34,10 +34,14 @@ static void xs_impl_delete(XenstoreImplState *s)
{
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 @@ static int write_str(XenstoreImplState *s, unsigned int dom_id,
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 @@ static XenstoreImplState *setup(void)
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 @@ static void test_xs_node_simple(void)
{
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 @@ static void test_xs_node_simple(void)
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 @@ static void test_xs_node_simple(void)
"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 @@ static void test_xs_node_simple(void)
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 @@ static void test_xs_node_simple(void)
"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 @@ static void test_xs_node_simple(void)
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 @@ static void test_xs_node_simple(void)
/* 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 @@ static void test_xs_node_simple(void)
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);
}