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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH 6 of 7 v2] blktap3/tapback: Introduce the tapback daemon
This patch introduces the core of the tapback daemon, the user space daemon
that acts as a device's back-end, essentially most of blkback in user space.
Similar to blkback, the daemon monitors XenStore for device creation/removal
requests and front-end state changes, and acts in response to them
(communicates to the front-end necessary information, switches the back-end's
state etc.).
The main issue in this patch is the following architectural change: in blktap2,
libxl spawns the tapdisk and then instructs it to connect to the shared ring
created by blktap. In blktap3, libxl spawns the tapdisk but the tapdisk just
sits there. When the tapback daemon detects that the front-end has switched
into the Initialised/Connected state, _it_ instructs the tapdisk to connect to
the shared ring (the one shared between tapdisk and the front-end). I don't
have the details of this change, but I don't see a reason why this shouldn't
work, or why it could be better compared to the blktap2 approach. Function
blkback_find_tapdisk is broken (see the FIXME in the code) works by chance,
once we decide on this architectural I'll fix it.
diff -r d332adcebe40 -r 407ebde55295 tools/blktap3/tapback/tapback.c
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/tools/blktap3/tapback/tapback.c Fri Jan 04 12:10:37 2013 +0000
@@ -0,0 +1,286 @@
+/*
+ * 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 core of the tapback daemon, the user space daemon
+ * that acts as a device's back-end.
+ */
+
+/*
+ * TODO Some of these includes may be useless.
+ * TODO Replace hard-coding strings with defines/const string.
+ */
+#include <stdlib.h>
+#include <stdarg.h>
+#include <assert.h>
+#include <syslog.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <libgen.h>
+#include <getopt.h>
+#include <sys/ioctl.h>
+#include <sys/mount.h>
+#include <syslog.h>
+
+#include "blktap3.h"
+#include "stdio.h" /* TODO tap-ctl.h needs to include stdio.h */
+#include "tap-ctl.h"
+#include "tapback.h"
+
+void tapback_log(int prio, const char *fmt, ...);
+void (*tapback_vlog) (int prio, const char *fmt, va_list ap);
+
+struct _blktap3_daemon blktap3_daemon;
+
+/**
+ * Read changes that occurred on the "backend/<backend name>" XenStore path
+ * or one of the front-end paths and act accordingly.
+ */
+static inline void
+tapback_read_watch(void)
+{
+ char **watch = NULL, *path = NULL, *token = NULL;
+ unsigned int n = 0;
+ int err = 0, _abort = 0;
+
+ /* read the change */
+ watch = xs_read_watch(blktap3_daemon.xs, &n);
+ path = watch[XS_WATCH_PATH];
+ token = watch[XS_WATCH_TOKEN];
+
+ /*
+ * TODO Put the body of "again:" into a loop instead of using goto.
+ */
+again:
+ if (!(blktap3_daemon.xst = xs_transaction_start(blktap3_daemon.xs))) {
+ WARN("error starting transaction\n");
+ goto fail;
+ }
+
+ /*
+ * The token indicates which XenStore watch triggered, the front-end one or
+ * the back-end one.
+ */
+ if (!strcmp(token, BLKTAP3_FRONTEND_TOKEN))
+ err = tapback_backend_handle_otherend_watch(path);
+ else if (!strcmp(token, BLKTAP3_BACKEND_TOKEN))
+ /*
+ * TODO The XenStore watch may trigger for a not yet
+ * discovered/understood reason. The result is
+ * tapback_backend_handle_backend_watch not doing anything interesting,
+ * it only checks the 'tapback-serial'.
+ */
+ err = tapback_backend_handle_backend_watch(path);
+ else {
+ WARN("invalid token \'%s\'\n", token);
+ err = EINVAL;
+ }
+
+ _abort = !!err;
+ if (_abort)
+ DBG("aborting transaction: %s\n", strerror(-err));
+
+ err = xs_transaction_end(blktap3_daemon.xs, blktap3_daemon.xst, _abort);
+ blktap3_daemon.xst = 0;
+ if (!err) {
+ err = -errno;
+ /*
+ * This is OK according to xs_transaction_end's semantics.
+ */
+ if (EAGAIN == errno)
+ goto again;
+ DBG("error ending transaction: %s\n", strerror(err));
+ }
+
+fail:
+ free(watch);
+ return;
+}
+
+static void
+tapback_backend_destroy(void)
+{
+ if (blktap3_daemon.xs) {
+ xs_daemon_close(blktap3_daemon.xs);
+ blktap3_daemon.xs = NULL;
+ }
+}
+
+/**
+ * Initializes the back-end descriptor. There is one back-end per tapback
+ * process. Also, it initiates a watch to XenStore on backend/<backend name>.
+ *
+ * @returns 0 on success, an error code otherwise
+ */
+static inline int
+tapback_backend_create(void)
+{
+ int err = -EINVAL;
+
+ TAILQ_INIT(&blktap3_daemon.devices);
+ blktap3_daemon.xst = XBT_NULL;
+
+ if (!(blktap3_daemon.xs = xs_daemon_open())) {
+ err = -EINVAL;
+ goto fail;
+ }
+
+ /*
+ * Watch the back-end.
+ */
+ if (!xs_watch(blktap3_daemon.xs, BLKTAP3_BACKEND_PATH,
+ BLKTAP3_BACKEND_TOKEN)) {
+ err = -errno;
+ goto fail;
+ }
+
+ return 0;
+
+fail:
+ tapback_backend_destroy();
+
+ return -err;
+}
+
+/**
+ * Runs the daemon.
+ *
+ * Watches backend/<backend name> and the front-end devices.
+ */
+static inline int
+tapback_backend_run(void)
+{
+ const int fd = xs_fileno(blktap3_daemon.xs);
+ int err;
+
+ do {
+ fd_set rfds;
+ int nfds = 0;
+
+ FD_ZERO(&rfds);
+ FD_SET(fd, &rfds);
+
+ /* poll the fd for changes in the XenStore path we're interested in */
+ if ((nfds = select(fd + 1, &rfds, NULL, NULL, NULL)) < 0) {
+ perror("error monitoring XenStore");
+ err = -errno;
+ break;
+ }
+
+ if (FD_ISSET(fd, &rfds))
+ tapback_read_watch();
+ } while (1);
+
+ return err;
+}
+
+static char *blkback_ident;
+
+static void
+blkback_vlog_fprintf(const int prio, const char * const fmt, va_list ap)
+{
+ const char *strprio[] = {
+ [LOG_DEBUG] = "DBG",
+ [LOG_INFO] = "INF",
+ [LOG_WARNING] = "WRN"
+ };
+
+ assert(prio >= 0);
+ BUG_ON(prio > sizeof(strprio) / sizeof(strprio[0]));
+ BUG_ON(!strprio[prio]);
+
+ fprintf(stderr, "%s[%s] ", blkback_ident, strprio[prio]);
+ vfprintf(stderr, fmt, ap);
+}
+
+/**
+ * Print tapback's usage instructions.
+ */
+static void
+usage(FILE * const stream, const char * const prog)
+{
+ assert(stream);
+ assert(prog);
+
+ fprintf(stream,
+ "usage: %s\n"
+ "\t[-D|--debug]\n"
+ "\t[-h|--help]\n", prog);
+}
+
+int main(int argc, char **argv)
+{
+ const char *prog = NULL;
+ int opt_debug = 0;
+ int err = 0;
+
+ prog = basename(argv[0]);
+
+ opt_debug = 0;
+
+ do {
+ const struct option longopts[] = {
+ {"help", 0, NULL, 'h'},
+ {"debug", 0, NULL, 'D'},
+ };
+ int c;
+
+ c = getopt_long(argc, argv, "h:D", longopts, NULL);
+ if (c < 0)
+ break;
+
+ switch (c) {
+ case 'h':
+ usage(stdout, prog);
+ return 0;
+ case 'D':
+ opt_debug = 1;
+ break;
+ case '?':
+ goto usage;
+ }
+ } while (1);
+
+ blkback_ident = BLKTAP3_BACKEND_TOKEN;
+ if (opt_debug)
+ tapback_vlog = blkback_vlog_fprintf;
+ else
+ openlog(blkback_ident, 0, LOG_DAEMON);
+
+ if (!opt_debug) {
+ if ((err = daemon(0, 0))) {
+ err = -errno;
+ goto fail;
+ }
+ }
+
+ if ((err = tapback_backend_create())) {
+ WARN("error creating blkback: %s\n", strerror(err));
+ goto fail;
+ }
+
+ err = tapback_backend_run();
+
+ tapback_backend_destroy();
+
+fail:
+ return err ? -err : 0;
+
+usage:
+ usage(stderr, prog);
+ return 1;
+}
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