[v2,2/3] tools: build qemu-vmsr-helper

diff --git a/docs/tools/index.rst b/docs/tools/index.rst
index 8e65ce0dfc7b..33ad438e86f6 100644
--- a/docs/tools/index.rst
+++ b/docs/tools/index.rst
@@ -16,3 +16,4 @@  command line utilities and other standalone programs.
    qemu-pr-helper
    qemu-trace-stap
    virtfs-proxy-helper
+   qemu-vmsr-helper
diff --git a/docs/tools/qemu-vmsr-helper.rst b/docs/tools/qemu-vmsr-helper.rst
new file mode 100644
index 000000000000..6ec87b49d962
--- /dev/null
+++ b/docs/tools/qemu-vmsr-helper.rst
@@ -0,0 +1,89 @@ 
+==================================
+QEMU virtual RAPL MSR helper
+==================================
+
+Synopsis
+--------
+
+**qemu-vmsr-helper** [*OPTION*]
+
+Description
+-----------
+
+Implements the virtual RAPL MSR helper for QEMU.
+
+Accessing the RAPL (Running Average Power Limit) MSR enables the RAPL powercap
+driver to advertise and monitor the power consumption or accumulated energy
+consumption of different power domains, such as CPU packages, DRAM, and other
+components when available.
+
+However those register are accesible under priviliged access (CAP_SYS_RAWIO).
+QEMU can use an external helper to access those priviliged register.
+
+:program:`qemu-vmsr-helper` is that external helper; it creates a listener
+socket which will accept incoming connections for communication with QEMU.
+
+If you want to run VMs in a setup like this, this helper should be started as a
+system service, and you should read the QEMU manual section on "RAPL MSR
+support" to find out how to configure QEMU to connect to the socket created by
+:program:`qemu-vmsr-helper`.
+
+After connecting to the socket, :program:`qemu-vmsr-helper` can
+optionally drop root privileges, except for those capabilities that
+are needed for its operation.
+
+:program:`qemu-vmsr-helper` can also use the systemd socket activation
+protocol.  In this case, the systemd socket unit should specify a
+Unix stream socket, like this::
+
+    [Socket]
+    ListenStream=/var/run/qemu-vmsr-helper.sock
+
+Options
+-------
+
+.. program:: qemu-vmsr-helper
+
+.. option:: -d, --daemon
+
+  run in the background (and create a PID file)
+
+.. option:: -q, --quiet
+
+  decrease verbosity
+
+.. option:: -v, --verbose
+
+  increase verbosity
+
+.. option:: -f, --pidfile=PATH
+
+  PID file when running as a daemon. By default the PID file
+  is created in the system runtime state directory, for example
+  :file:`/var/run/qemu-vmsr-helper.pid`.
+
+.. option:: -k, --socket=PATH
+
+  path to the socket. By default the socket is created in
+  the system runtime state directory, for example
+  :file:`/var/run/qemu-vmsr-helper.sock`.
+
+.. option:: -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE]
+
+  .. include:: ../qemu-option-trace.rst.inc
+
+.. option:: -u, --user=USER
+
+  user to drop privileges to
+
+.. option:: -g, --group=GROUP
+
+  group to drop privileges to
+
+.. option:: -h, --help
+
+  Display a help message and exit.
+
+.. option:: -V, --version
+
+  Display version information and exit.
diff --git a/meson.build b/meson.build
index dcef8b1e7911..d30a7a09d46f 100644
--- a/meson.build
+++ b/meson.build
@@ -3950,6 +3950,11 @@  if have_tools
                dependencies: [authz, crypto, io, qom, qemuutil,
                               libcap_ng, mpathpersist],
                install: true)
+
+    executable('qemu-vmsr-helper', files('tools/i386/qemu-vmsr-helper.c'),
+               dependencies: [authz, crypto, io, qom, qemuutil,
+                              libcap_ng, mpathpersist],
+               install: true)
   endif
 
   if have_ivshmem
diff --git a/tools/i386/qemu-vmsr-helper.c b/tools/i386/qemu-vmsr-helper.c
new file mode 100644
index 000000000000..1d82a2753e44
--- /dev/null
+++ b/tools/i386/qemu-vmsr-helper.c
@@ -0,0 +1,507 @@ 
+/*
+ * Privileged RAPL MSR helper commands for QEMU
+ *
+ * Copyright (C) 2023 Red Hat, Inc. <aharivel@redhat.com>
+ *
+ * Author: Anthony Harivel <aharivel@redhat.com>
+ *
+ * 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; under version 2 of the License.
+ *
+ * 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, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "qemu/osdep.h"
+#include <getopt.h>
+#include <stdbool.h>
+#include <sys/ioctl.h>
+#ifdef CONFIG_LIBCAP_NG
+#include <cap-ng.h>
+#endif
+#include <pwd.h>
+#include <grp.h>
+
+#include "qemu/help-texts.h"
+#include "qapi/error.h"
+#include "qemu/cutils.h"
+#include "qemu/main-loop.h"
+#include "qemu/module.h"
+#include "qemu/error-report.h"
+#include "qemu/config-file.h"
+#include "qemu-version.h"
+#include "qapi/error.h"
+#include "qemu/error-report.h"
+#include "qemu/log.h"
+#include "qemu/systemd.h"
+#include "qapi/util.h"
+#include "io/channel.h"
+#include "io/channel-socket.h"
+#include "trace/control.h"
+#include "qemu-version.h"
+#include "rapl-msr-index.h"
+
+#define MAX_PATH_LEN 50
+#define MAX_LINE_LEN 500
+#define PR_OUT_FIXED_PARAM_SIZE 24
+
+static char *socket_path;
+static char *pidfile;
+static enum { RUNNING, TERMINATE, TERMINATING } state;
+static QIOChannelSocket *server_ioc;
+static int server_watch;
+static int num_active_sockets = 1;
+static int noisy;
+static int verbose;
+
+#ifdef CONFIG_LIBCAP_NG
+static int uid = -1;
+static int gid = -1;
+#endif
+
+static void compute_default_paths(void)
+{
+    g_autofree char *state = qemu_get_local_state_dir();
+
+    socket_path = g_build_filename(state, "run", "qemu-vmsr-helper.sock", NULL);
+    pidfile = g_build_filename(state, "run", "qemu-vmsr-helper.pid", NULL);
+}
+
+/*
+ * Check if the TID that request the MSR read
+ * belongs to the peer. It should a TID of a vCPU.
+ */
+static bool is_tid_present(pid_t pid, pid_t tid)
+{
+    char pidStr[20];
+    char tidStr[20];
+
+    snprintf(pidStr, sizeof(pidStr), "%d", pid);
+    snprintf(tidStr, sizeof(tidStr), "%d", tid);
+
+    char pidPath[256];
+    char tidPath[256];
+
+    snprintf(pidPath, sizeof(pidPath), "/proc/%s", pidStr);
+    snprintf(tidPath, sizeof(tidPath), "/proc/%s/task/%s", pidStr, tidStr);
+
+    /* Check if the TID directory exists within the PID directory */
+    if (access(tidPath, F_OK) == 0) {
+        return true;
+    }
+
+    return false;
+}
+
+/*
+ * Only the RAPL MSR in target/i386/cpu.h are allowed
+ */
+static bool is_msr_allowed(uint32_t reg)
+{
+    switch (reg) {
+    case MSR_RAPL_POWER_UNIT:
+    case MSR_PKG_POWER_LIMIT:
+    case MSR_PKG_ENERGY_STATUS:
+    case MSR_PKG_POWER_INFO:
+        return true;
+    default:
+        return false;
+    }
+}
+
+static uint64_t vmsr_read_msr(uint32_t reg, unsigned int cpu_id)
+{
+    int fd;
+    uint64_t data;
+
+    char path[MAX_PATH_LEN];
+    snprintf(path, MAX_PATH_LEN, "/dev/cpu/%u/msr", cpu_id);
+
+    fd = open(path , O_RDONLY);
+    if (fd < 0) {
+        return 0;
+    }
+    if (pread(fd, &data, sizeof data, reg) != sizeof data) {
+        data = 0;
+    }
+
+    close(fd);
+    return data;
+}
+
+static void usage(const char *name)
+{
+    (printf) (
+"Usage: %s [OPTIONS] FILE\n"
+"Virtual RAPL MSR helper program for QEMU\n"
+"\n"
+"  -h, --help                display this help and exit\n"
+"  -V, --version             output version information and exit\n"
+"\n"
+"  -d, --daemon              run in the background\n"
+"  -f, --pidfile=PATH        PID file when running as a daemon\n"
+"                            (default '%s')\n"
+"  -k, --socket=PATH         path to the unix socket\n"
+"                            (default '%s')\n"
+"  -T, --trace [[enable=]<pattern>][,events=<file>][,file=<file>]\n"
+"                            specify tracing options\n"
+#ifdef CONFIG_LIBCAP_NG
+"  -u, --user=USER           user to drop privileges to\n"
+"  -g, --group=GROUP         group to drop privileges to\n"
+#endif
+"\n"
+QEMU_HELP_BOTTOM "\n"
+    , name, pidfile, socket_path);
+}
+
+static void version(const char *name)
+{
+    printf(
+"%s " QEMU_FULL_VERSION "\n"
+"Written by Anthony Harivel.\n"
+"\n"
+QEMU_COPYRIGHT "\n"
+"This is free software; see the source for copying conditions.  There is NO\n"
+"warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.\n"
+    , name);
+}
+
+typedef struct VMSRHelperClient {
+    QIOChannelSocket *ioc;
+    Coroutine *co;
+    int fd;
+} VMSRHelperClient;
+
+static void coroutine_fn vh_co_entry(void *opaque)
+{
+    VMSRHelperClient *client = opaque;
+    uint64_t vmsr;
+    uint32_t request[3];
+    struct ucred cred;
+    int r;
+    Error *local_err = NULL;
+
+    qio_channel_set_blocking(QIO_CHANNEL(client->ioc),
+                             false, NULL);
+
+    qio_channel_set_follow_coroutine_ctx(QIO_CHANNEL(client->ioc), true);
+
+    /*
+     * Check peer credentials
+     * Only QEMU PID/TID are allowed
+     */
+    qio_channel_get_peercred(QIO_CHANNEL(client->ioc), &cred, &local_err);
+
+    if (cred.pid == 0) {
+        if (local_err != NULL) {
+            error_report_err(local_err);
+        }
+        error_report("Failed to get peer credentials");
+        goto out;
+    }
+
+    /*
+     * Read the requested MSR
+     * Only RAPL MSR in rapl-msr-index.h is allowed
+     */
+    r = qio_channel_read_all(QIO_CHANNEL(client->ioc),
+                             (char *) &request, sizeof(request), NULL);
+    if (!is_msr_allowed(request[0]) || r < 0) {
+        error_report("Read request fail: %d, %d", request[0], request[1]);
+        goto out;
+    }
+
+    vmsr = vmsr_read_msr(request[0], request[1]);
+
+    if (!is_tid_present(cred.pid, request[2])) {
+        error_report("requested TID not in peer PID");
+        vmsr = 0;
+    }
+
+    r = qio_channel_write_all(QIO_CHANNEL(client->ioc),
+                         (char *) &vmsr, sizeof(vmsr), NULL);
+    if (r < 0) {
+        error_report("write vmsr failed");
+        goto out;
+    }
+
+out:
+    object_unref(OBJECT(client->ioc));
+    g_free(client);
+}
+
+static gboolean accept_client(QIOChannel *ioc,
+                              GIOCondition cond,
+                              gpointer opaque)
+{
+    QIOChannelSocket *cioc;
+    VMSRHelperClient *vmsrh;
+
+    cioc = qio_channel_socket_accept(QIO_CHANNEL_SOCKET(ioc),
+                                     NULL);
+    if (!cioc) {
+        return TRUE;
+    }
+
+    vmsrh = g_new(VMSRHelperClient, 1);
+    vmsrh->ioc = cioc;
+    vmsrh->fd = -1;
+    vmsrh->co = qemu_coroutine_create(vh_co_entry, vmsrh);
+    qemu_coroutine_enter(vmsrh->co);
+
+    return TRUE;
+}
+
+static void termsig_handler(int signum)
+{
+    qatomic_cmpxchg(&state, RUNNING, TERMINATE);
+    qemu_notify_event();
+}
+
+static void close_server_socket(void)
+{
+    assert(server_ioc);
+
+    g_source_remove(server_watch);
+    server_watch = -1;
+    object_unref(OBJECT(server_ioc));
+    num_active_sockets--;
+}
+
+#ifdef CONFIG_LIBCAP_NG
+static int drop_privileges(void)
+{
+    /* clear all capabilities */
+    capng_clear(CAPNG_SELECT_BOTH);
+
+    if (capng_update(CAPNG_ADD, CAPNG_EFFECTIVE | CAPNG_PERMITTED,
+                     CAP_SYS_RAWIO) < 0) {
+        return -1;
+    }
+
+    /*
+     * Change user/group id, retaining the capabilities.
+     * Because file descriptors are passed via SCM_RIGHTS,
+     * we don't need supplementary groups (and in fact the helper
+     * can run as "nobody").
+     */
+    if (capng_change_id(uid != -1 ? uid : getuid(),
+                        gid != -1 ? gid : getgid(),
+                        CAPNG_DROP_SUPP_GRP | CAPNG_CLEAR_BOUNDING)) {
+        return -1;
+    }
+
+    return 0;
+}
+#endif
+
+int main(int argc, char **argv)
+{
+    const char *sopt = "hVk:f:dT:u:g:vq";
+    struct option lopt[] = {
+        { "help", no_argument, NULL, 'h' },
+        { "version", no_argument, NULL, 'V' },
+        { "socket", required_argument, NULL, 'k' },
+        { "pidfile", required_argument, NULL, 'f' },
+        { "daemon", no_argument, NULL, 'd' },
+        { "trace", required_argument, NULL, 'T' },
+        { "user", required_argument, NULL, 'u' },
+        { "group", required_argument, NULL, 'g' },
+        { "verbose", no_argument, NULL, 'v' },
+        { "quiet", no_argument, NULL, 'q' },
+        { NULL, 0, NULL, 0 }
+    };
+    int opt_ind = 0;
+    int loglevel = 1;
+    int quiet = 0;
+    int ch;
+    Error *local_err = NULL;
+    bool daemonize = false;
+    bool pidfile_specified = false;
+    bool socket_path_specified = false;
+    unsigned socket_activation;
+
+    struct sigaction sa_sigterm;
+    memset(&sa_sigterm, 0, sizeof(sa_sigterm));
+    sa_sigterm.sa_handler = termsig_handler;
+    sigaction(SIGTERM, &sa_sigterm, NULL);
+    sigaction(SIGINT, &sa_sigterm, NULL);
+    sigaction(SIGHUP, &sa_sigterm, NULL);
+
+    signal(SIGPIPE, SIG_IGN);
+
+    error_init(argv[0]);
+    module_call_init(MODULE_INIT_TRACE);
+    module_call_init(MODULE_INIT_QOM);
+    qemu_add_opts(&qemu_trace_opts);
+    qemu_init_exec_dir(argv[0]);
+
+    compute_default_paths();
+
+    while ((ch = getopt_long(argc, argv, sopt, lopt, &opt_ind)) != -1) {
+        switch (ch) {
+        case 'k':
+            g_free(socket_path);
+            socket_path = g_strdup(optarg);
+            socket_path_specified = true;
+            if (socket_path[0] != '/') {
+                error_report("socket path must be absolute");
+                exit(EXIT_FAILURE);
+            }
+            break;
+        case 'f':
+            g_free(pidfile);
+            pidfile = g_strdup(optarg);
+            pidfile_specified = true;
+            break;
+#ifdef CONFIG_LIBCAP_NG
+        case 'u': {
+            unsigned long res;
+            struct passwd *userinfo = getpwnam(optarg);
+            if (userinfo) {
+                uid = userinfo->pw_uid;
+            } else if (qemu_strtoul(optarg, NULL, 10, &res) == 0 &&
+                       (uid_t)res == res) {
+                uid = res;
+            } else {
+                error_report("invalid user '%s'", optarg);
+                exit(EXIT_FAILURE);
+            }
+            break;
+        }
+        case 'g': {
+            unsigned long res;
+            struct group *groupinfo = getgrnam(optarg);
+            if (groupinfo) {
+                gid = groupinfo->gr_gid;
+            } else if (qemu_strtoul(optarg, NULL, 10, &res) == 0 &&
+                       (gid_t)res == res) {
+                gid = res;
+            } else {
+                error_report("invalid group '%s'", optarg);
+                exit(EXIT_FAILURE);
+            }
+            break;
+        }
+#else
+        case 'u':
+        case 'g':
+            error_report("-%c not supported by this %s", ch, argv[0]);
+            exit(1);
+#endif
+        case 'd':
+            daemonize = true;
+            break;
+        case 'q':
+            quiet = 1;
+            break;
+        case 'v':
+            ++loglevel;
+            break;
+        case 'T':
+            trace_opt_parse(optarg);
+            break;
+        case 'V':
+            version(argv[0]);
+            exit(EXIT_SUCCESS);
+            break;
+        case 'h':
+            usage(argv[0]);
+            exit(EXIT_SUCCESS);
+            break;
+        case '?':
+            error_report("Try `%s --help' for more information.", argv[0]);
+            exit(EXIT_FAILURE);
+        }
+    }
+
+    /* set verbosity */
+    noisy = !quiet && (loglevel >= 3);
+    verbose = quiet ? 0 : MIN(loglevel, 3);
+
+    if (!trace_init_backends()) {
+        exit(EXIT_FAILURE);
+    }
+    trace_init_file();
+    qemu_set_log(LOG_TRACE, &error_fatal);
+
+    socket_activation = check_socket_activation();
+    if (socket_activation == 0) {
+        SocketAddress saddr;
+        saddr = (SocketAddress){
+            .type = SOCKET_ADDRESS_TYPE_UNIX,
+            .u.q_unix.path = socket_path,
+        };
+        server_ioc = qio_channel_socket_new();
+        if (qio_channel_socket_listen_sync(server_ioc, &saddr,
+                                           1, &local_err) < 0) {
+            object_unref(OBJECT(server_ioc));
+            error_report_err(local_err);
+            return 1;
+        }
+    } else {
+        /* Using socket activation - check user didn't use -p etc. */
+        if (socket_path_specified) {
+            error_report("Unix socket can't be set when \
+                        using socket activation");
+            exit(EXIT_FAILURE);
+        }
+
+        /* Can only listen on a single socket.  */
+        if (socket_activation > 1) {
+            error_report("%s does not support socket activation \
+                        with LISTEN_FDS > 1",
+                        argv[0]);
+            exit(EXIT_FAILURE);
+        }
+        server_ioc = qio_channel_socket_new_fd(FIRST_SOCKET_ACTIVATION_FD,
+                                               &local_err);
+        if (server_ioc == NULL) {
+            error_reportf_err(local_err,
+                              "Failed to use socket activation: ");
+            exit(EXIT_FAILURE);
+        }
+    }
+
+    qemu_init_main_loop(&error_fatal);
+
+    server_watch = qio_channel_add_watch(QIO_CHANNEL(server_ioc),
+                                         G_IO_IN,
+                                         accept_client,
+                                         NULL, NULL);
+
+    if (daemonize) {
+        if (daemon(0, 0) < 0) {
+            error_report("Failed to daemonize: %s", strerror(errno));
+            exit(EXIT_FAILURE);
+        }
+    }
+
+    if (daemonize || pidfile_specified) {
+        qemu_write_pidfile(pidfile, &error_fatal);
+    }
+
+#ifdef CONFIG_LIBCAP_NG
+    if (drop_privileges() < 0) {
+        error_report("Failed to drop privileges: %s", strerror(errno));
+        exit(EXIT_FAILURE);
+    }
+#endif
+
+    state = RUNNING;
+    do {
+        main_loop_wait(false);
+        if (state == TERMINATE) {
+            state = TERMINATING;
+            close_server_socket();
+        }
+    } while (num_active_sockets > 0);
+
+    exit(EXIT_SUCCESS);
+}
diff --git a/tools/i386/rapl-msr-index.h b/tools/i386/rapl-msr-index.h
new file mode 100644
index 000000000000..9a7118639ae3
--- /dev/null
+++ b/tools/i386/rapl-msr-index.h
@@ -0,0 +1,28 @@ 
+/*
+ * Allowed list of MSR for Privileged RAPL MSR helper commands for QEMU
+ *
+ * Copyright (C) 2023 Red Hat, Inc. <aharivel@redhat.com>
+ *
+ * Author: Anthony Harivel <aharivel@redhat.com>
+ *
+ * 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; under version 2 of the License.
+ *
+ * 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, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+ * Should stay in sync with the RAPL MSR
+ * in target/i386/cpu.h
+ */
+#define MSR_RAPL_POWER_UNIT             0x00000606
+#define MSR_PKG_POWER_LIMIT             0x00000610
+#define MSR_PKG_ENERGY_STATUS           0x00000611
+#define MSR_PKG_POWER_INFO              0x00000614