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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH 4 of 8 v3] blktap3/tapback: Introduce back-end XenStore path handler
This patch introduces the handler executed when the back-end XenStore path gets
modified. A back-end XenStore path is modified as a result of a device
creation/removal request. The device is created/removed depending on whether
its path exists/does not exist in XenStore.
Creating a device comprises creating the in-memory representation of it and
adding it to the list of devices, locating the tapdisk designated to serve
this VBD, and setting a XenStore watch to the front-end path of the
newly-created device.
Deleting a device comprises removing that XenStore watch and deallocating its
in-memory representation.
Signed-off-by: Thanos Makatos <thanos.makatos@xxxxxxxxxx>
---
Changed since v2:
* Removed the "serial" thing.
* Function tapback_backend_handle_backend_watch doesn't always have to scan
* the entire back-end sub-tree, this is left as a future optimisation
(relevant comment inside code updated).
* Replaced mprintf with asprintf.
* Check for failures in tapback_backend_scan.
* Minor code and whitespace clean up.
* The tapback daemon searches for a suitable tapdisk and creates one if none
is found. It also kills it when the front-end disconnects.
diff --git a/tools/blktap3/tapback/backend.c b/tools/blktap3/tapback/backend.c
new file mode 100644
--- /dev/null
+++ b/tools/blktap3/tapback/backend.c
@@ -0,0 +1,503 @@
+/*
+ * Copyright (C) 2012 Citrix Ltd.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
+ * USA.
+ *
+ * This file contains the handler executed when the back-end XenStore path gets
+ * modified.
+ */
+
+#include "tapback.h"
+#include "xenstore.h"
+#include "tap-ctl-info.h"
+
+/**
+ * Removes the XenStore watch from the front-end.
+ *
+ * @param device the VBD whose front-end XenStore path should stop being
+ * watched
+ */
+static void
+tapback_device_unwatch_frontend_state(vbd_t * const device)
+{
+ assert(device);
+
+ if (device->frontend_state_path)
+ xs_unwatch(blktap3_daemon.xs, device->frontend_state_path,
+ BLKTAP3_FRONTEND_TOKEN);
+
+ free(device->frontend_state_path);
+ device->frontend_state_path = NULL;
+}
+
+/**
+ * Destroys and deallocates the back-end part of a VBD.
+ *
+ * @param device the VBD to destroy
+ */
+static void
+tapback_backend_destroy_device(vbd_t * const device)
+{
+ int err;
+
+ assert(device);
+
+ TAILQ_REMOVE(&blktap3_daemon.devices, device, backend_entry);
+
+ tapback_device_unwatch_frontend_state(device);
+
+ /*
+ * kill the tapdisk
+ */
+ err = tap_ctl_destroy(device->tap.pid, device->tap.minor, 0, NULL);
+ if (err)
+ WARN("failed to destroy tapdisk %d/%d: %s (error ignored)\n",
+ device->tap.pid, device->tap.minor, strerror(err));
+
+ free(device->frontend_path);
+ free(device->name);
+
+ /*
+ * XXX device->bdev is expected to have been freed
+ */
+
+ free(device);
+}
+
+/**
+ * Retrieves the tapdisk designated to serve this device, storing this
+ * information in the supplied VBD handle.
+ *
+ * @param param <type>:/path/to/file
+ * @param tap output parameter that receives the tapdisk process information.
+ * The parameter is undefined when the function returns a non-zero value.
+ * @returns 0 if a suitable tapdisk is found, ESRCH if no suitable tapdisk is
+ * found, and an error code in case of error
+ *
+ * TODO rename function
+ *
+ * XXX Only called by blkback_probe.
+ */
+static inline int
+blkback_find_tapdisk(const char *params, tap_list_t *tap)
+{
+ struct tqh_tap_list list;
+ tap_list_t *_tap;
+ int err;
+ char *type;
+ char *path;
+ char *_params = NULL;
+
+ assert(params);
+ assert(tap);
+
+ _params = strdup(params);
+ if (!_params) {
+ WARN("failed to allocate memory\n");
+ err = ENOMEM;
+ goto out;
+ }
+
+ type = strtok(_params, ":");
+ if (type) {
+ path = strtok(NULL, ":");
+ }
+ if (!type || !path) {
+ WARN("malformed params \'%s\'\n", params);
+ err = EINVAL;
+ goto out;
+ }
+
+ err = tap_ctl_list(&list);
+ if (err) {
+ WARN("error listing tapdisks: %s\n", strerror(err));
+ goto out;
+ }
+
+ err = ESRCH;
+ if (!TAILQ_EMPTY(&list)) {
+ tap_list_for_each_entry(_tap, &list) {
+ if (_tap->type && !strcmp(_tap->type, type) && _tap->path
+ && !strcmp(_tap->path, path)) {
+ err = 0;
+ memcpy(tap, _tap, sizeof(tap));
+ break;
+ }
+ }
+ tap_ctl_list_free(&list);
+ } else
+ DBG("no tapdisks\n");
+
+out:
+ free(_params);
+ return err;
+}
+
+/**
+ * Creates a device and adds it to the list of devices.
+ * Initiates a XenStore watch to the front-end state.
+ *
+ * Creating the device implies initializing the handle and retrieving all the
+ * information of the tapdisk serving this VBD.
+ *
+ * @param domid the ID of the domain where the VBD is created
+ * @param name the name of the device
+ * @returns 0 on success, an error code otherwise
+ */
+static inline int
+tapback_backend_create_device(const domid_t domid, const char * const name)
+{
+ vbd_t *device = NULL;
+ int err = 0;
+ char *params = NULL;
+
+ assert(name);
+
+ DBG("creating device %d/%s\n", domid, name);
+
+ if (!(device = calloc(1, sizeof(*device)))) {
+ WARN("error allocating memory\n");
+ err = -errno;
+ goto out;
+ }
+
+ device->domid = domid;
+
+ TAILQ_INSERT_TAIL(&blktap3_daemon.devices, device, backend_entry);
+
+ if (!(device->name = strdup(name))) {
+ err = -errno;
+ goto out;
+ }
+
+ /*
+ * Get the front-end path from XenStore. We need this to talk to the
+ * front-end.
+ */
+ if (!(device->frontend_path = tapback_device_read(device, "frontend"))) {
+ err = errno;
+ WARN("failed to read front-end path: %s\n", strerror(err));
+ goto out;
+ }
+
+ /*
+ * Get the file path backing the VBD.
+ */
+ params = tapback_xs_read(blktap3_daemon.xs, blktap3_daemon.xst,
+ "%s/%d/%s/params", BLKTAP3_BACKEND_PATH, domid, name);
+ if (!params) {
+ err = errno;
+ WARN("failed to read backing file: %s\n", strerror(err));
+ goto out;
+ }
+ DBG("need to find tapdisk serving \'%s\'\n", params);
+
+ err = blkback_find_tapdisk(params, &device->tap);
+ if (!err) {
+ DBG("found tapdisk %d/%d\n", device->tap.pid, device->tap.minor);
+ } else if (err == ESRCH) {
+ /* FIXME replace with tap_ctl_create */
+ DBG("no such tapdisk\n");
+ err = tap_ctl_spawn();
+ if (err <= 0) {
+ WARN("failed to create tapdisk: %s\n", strerror(err));
+ goto out;
+ }
+ device->tap.pid = err;
+ DBG("spawned tapdisk %d\n", device->tap.pid);
+ device->tap.minor = 0;
+ err = tap_ctl_attach(device->tap.pid, device->tap.minor);
+ if (err) {
+ WARN("failed to attach tapdisk: %s\n", strerror(err));
+ goto out;
+ }
+ DBG("attached tapdisk %d\n", device->tap.pid);
+ err = tap_ctl_open(device->tap.pid, 0, params, 0, -1, NULL);
+ if (err) {
+ WARN("failed to open tapdisk: %s\n", strerror(err));
+ goto out;
+ }
+ DBG("opened tapdisk %d\n", device->tap.pid);
+ } else {
+ WARN("error looking for tapdisk: %s\n", strerror(err));
+ goto out;
+ }
+
+ /*
+ * get the VBD parameters from the tapdisk
+ */
+ if ((err = tap_ctl_info(device->tap.pid, 0, &device->sectors,
+ &device->sector_size, &device->info))) {
+ WARN("error retrieving disk characteristics: %s\n", strerror(-err));
+ goto out;
+ }
+
+ DBG("got %d-%d with tapdev %d/%d\n", device->domid, device->devid,
+ device->tap.pid, device->tap.minor);
+
+ /*
+ * Finally, watch the front-end path in XenStore for changes, i.e.
+ * /local/domain/<domid>/device/vbd/<devname>/state
+ * After this, we wait for the front-end to switch state to continue with
+ * the initialisation.
+ */
+ if (asprintf(&device->frontend_state_path, "%s/state",
+ device->frontend_path) == -1) {
+ /* TODO log error */
+ err = -errno;
+ goto out;
+ }
+ assert(device->frontend_state_path);
+
+ /*
+ * We use the same token for all front-end watches. We don't have to use a
+ * unique token for each front-end watch because when a front-end watch
+ * fires we are given the XenStore path that changed.
+ */
+ if (!xs_watch(blktap3_daemon.xs, device->frontend_state_path,
+ BLKTAP3_FRONTEND_TOKEN)) {
+ free(device->frontend_state_path);
+ err = -errno;
+ goto out;
+ }
+
+out:
+ if (err) {
+ WARN("error creating device: domid=%d name=%s err=%d (%s)\n",
+ domid, name, err, strerror(err));
+ if (device)
+ tapback_backend_destroy_device(device);
+ }
+ free(params);
+ return err;
+}
+
+/**
+ * Creates (removes) a device depending on the existence (non-existence) of the
+ * "backend/<backend name>/@domid/@devname" XenStore path.
+ *
+ * @param domid the ID of the domain where the VBD is created
+ * @param devname device name
+ * @returns 0 on success, an error code otherwise
+ */
+static int
+tapback_backend_probe_device(const domid_t domid, const char * const devname)
+{
+ int should_exist = 0, create = 0, remove = 0;
+ vbd_t *device = NULL;
+ char * s = NULL;
+
+ assert(devname);
+
+ DBG("probing device domid=%d name=%s\n", domid, devname);
+
+ /*
+ * Ask XenStore if the device _should_ exist.
+ */
+ s = tapback_xs_read(blktap3_daemon.xs, blktap3_daemon.xst, "%s/%d/%s",
+ BLKTAP3_BACKEND_PATH, domid, devname);
+ should_exist = s != NULL;
+ free(s);
+
+ /*
+ * Search the device list for this specific device.
+ */
+ tapback_backend_find_device(device,
+ device->domid == domid && !strcmp(device->name, devname));
+
+ /*
+ * If XenStore says that the device should exist but it's not in our
device
+ * list, we must create it. If it's the other way round, this is a removal.
+ */
+ remove = device && !should_exist;
+ create = !device && should_exist;
+
+ DBG("should exist=%d device=%p remove=%d create=%d\n",
+ should_exist, device, remove, create);
+
+ if (!create && !remove) {
+ /*
+ * A watch has triggered on a path we're not interested in.
+ * TODO Check if we can avoid probing the device completely based on
+ * the path that triggered.
+ */
+ DBG("spurious XenStore watch triggered on back-end\n");
+ return 0;
+ }
+
+ /*
+ * Remember that remove and create may both be true at the same time, as
+ * this indicates that the device has been removed and re-created too fast.
+ * In this case, we too need to remove and re-create the device,
+ * respectively.
+ */
+
+ if (remove)
+ tapback_backend_destroy_device(device);
+
+ if (create) {
+ const int err = tapback_backend_create_device(domid, devname);
+ if (0 != err) {
+ WARN("error creating device %s on domain %d: %s\n", devname, domid,
+ strerror(err));
+ return err;
+ }
+ }
+
+ return 0;
+}
+
+/**
+ * Scans XenStore for all blktap3 devices and probes each one of them.
+ *
+ * XXX Only called by tapback_backend_handle_backend_watch.
+ */
+static int
+tapback_backend_scan(void)
+{
+ vbd_t *device = NULL, *next = NULL;
+ unsigned int i = 0, j = 0, n = 0, m = 0;
+ char **dir = NULL;
+ int err = 0;
+
+ DBG("scanning the back-end\n");
+
+ /*
+ * scrap all non-existent devices
+ * FIXME Why do we do this?
+ * FIXME Is this costly?
+ */
+
+ tapback_backend_for_each_device(device, next) {
+ err = tapback_backend_probe_device(device->domid, device->name);
+ if (err) {
+ WARN("error probing device %s of domain %d: %s\n", device->name,
+ device->domid, strerror(err));
+ /* TODO Should we fail in this case of keep probing? */
+ goto out;
+ }
+ }
+
+ /*
+ * probe the new ones
+ *
+ * FIXME We're checking each and every device in each and every domain,
+ * could there be a performance issue in the presence of many VMs/VBDs?
+ * (e.g. boot-storm)
+ */
+ if (!(dir = xs_directory(blktap3_daemon.xs, blktap3_daemon.xst,
+ BLKTAP3_BACKEND_PATH, &n))) {
+ err = errno;
+ if (err == ENOENT)
+ err = 0;
+ else
+ WARN("error listing %s: %s\n", BLKTAP3_BACKEND_PATH,
+ strerror(err));
+ goto out;
+ }
+
+ DBG("probing %d domains\n", n);
+
+ for (i = 0; i < n; i++) { /* for each domain */
+ char *path = NULL, **sub = NULL, *end = NULL;
+ domid_t domid = 0;
+
+ /*
+ * Get the domain ID.
+ */
+ domid = strtoul(dir[i], &end, 0);
+ if (*end != 0 || end == dir[i])
+ continue;
+
+ /*
+ * Read the devices of this domain.
+ */
+ if (asprintf(&path, "%s/%d", BLKTAP3_BACKEND_PATH, domid) == -1) {
+ /* TODO log error */
+ err = errno;
+ goto out;
+ }
+ sub = xs_directory(blktap3_daemon.xs, blktap3_daemon.xst, path, &m);
+ err = errno;
+ free(path);
+
+ if (!sub) {
+ WARN("error listing %s: %s\n", path, strerror(err));
+ goto out;
+ }
+
+ /*
+ * Probe each device.
+ */
+ for (j = 0; j < m; j++) {
+ err = tapback_backend_probe_device(domid, sub[j]);
+ if (err) {
+ WARN("error probing device %s of domain %d: %s\n", sub[j],
+ domid, strerror(err));
+ goto out;
+ }
+ }
+
+ free(sub);
+ }
+
+out:
+ free(dir);
+ return err;
+}
+
+int
+tapback_backend_handle_backend_watch(char * const path)
+{
+ char *s = NULL, *end = NULL, *name = NULL;
+ domid_t domid = 0;
+
+ assert(path);
+
+ DBG("handling watch triggered on path \'%s\'\n", path);
+
+ s = strtok(path, "/");
+ assert(!strcmp(s, XENSTORE_BACKEND));
+ if (!(s = strtok(NULL, "/")))
+ return tapback_backend_scan();
+
+ assert(!strcmp(s, BLKTAP3_BACKEND_NAME));
+ if (!(s = strtok(NULL, "/")))
+ return tapback_backend_scan();
+
+ domid = strtoul(s, &end, 0);
+ if (*end != 0 || end == s) {
+ WARN("invalid domain ID \'%s\'\n", s);
+ return EINVAL;
+ }
+
+ /*
+ * TODO Optimisation: since we know which domain changed, we don't have to
+ * scan the whole thing. Add the domid as an optional parameter to
+ * tapback_backend_scan.
+ */
+ if (!(name = strtok(NULL, "/")))
+ return tapback_backend_scan();
+
+ /*
+ * Create or remove a specific device.
+ *
+ * TODO tapback_backend_probe_device reads xenstore again to see if the
+ * device should exist, but we already know that in the current function.
+ * Optimise this case.
+ */
+ return tapback_backend_probe_device(domid, name);
+}
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