diff --git a/docs/specs/tpm.txt b/docs/specs/tpm.txt
index 914daaceeafa4ad4010f8837364d283c6a67d20b..d1d71571e982ddd921766270a5fe3501967e7e23 100644
--- a/docs/specs/tpm.txt
+++ b/docs/specs/tpm.txt
@@ -121,3 +121,82 @@ crw-------. 1 root root 10, 224 Jul 11 10:11 /dev/tpm0
 PCR-00: 35 4E 3B CE 23 9F 38 59 ...
 ...
 PCR-23: 00 00 00 00 00 00 00 00 ...
+
+
+== The QEMU TPM emulator device ==
+
+The TPM emulator device uses an external TPM emulator called 'swtpm' for
+sending TPM commands to and receiving responses from. The swtpm program
+must have been started before trying to access it through the TPM emulator
+with QEMU.
+
+The TPM emulator implements a command channel for transferring TPM commands
+and responses as well as a control channel over which control commands can
+be sent. The specification for the control channel can be found here:
+
+https://github.com/stefanberger/swtpm/blob/master/man/man3/swtpm_ioctls.pod
+
+
+The control channel serves the purpose of resetting, initializing, and
+migrating the TPM state, among other things.
+
+The swtpm program behaves like a hardware TPM and therefore needs to be
+initialized by the firmware running inside the QEMU virtual machine.
+One necessary step for initializing the device is to send the TPM_Startup
+command to it. SeaBIOS, for example, has been instrumented to initialize
+a TPM 1.2 or TPM 2 device using this command.
+
+
+QEMU files related to the TPM emulator device:
+ - hw/tpm/tpm_emulator.c
+ - hw/tpm/tpm_util.c
+ - hw/tpm/tpm_util.h
+
+
+The following commands start the swtpm with a UnixIO control channel over
+a socket interface. They do not need to be run as root.
+
+mkdir /tmp/mytpm1
+swtpm socket --tpmstate dir=/tmp/mytpm1 \
+  --ctrl type=unixio,path=/tmp/mytpm1/swtpm-sock \
+  --log level=20
+
+Command line to start QEMU with the TPM emulator device communicating with
+the swtpm:
+
+qemu-system-x86_64 -display sdl -enable-kvm \
+  -m 1024 -boot d -bios bios-256k.bin -boot menu=on \
+  -chardev socket,id=chrtpm,path=/tmp/mytpm1/swtpm-sock \
+  -tpmdev emulator,id=tpm0,chardev=chrtpm \
+  -device tpm-tis,tpmdev=tpm0 test.img
+
+
+In case SeaBIOS is used as firmware, it should show the TPM menu item
+after entering the menu with 'ESC'.
+
+Select boot device:
+1. DVD/CD [ata1-0: QEMU DVD-ROM ATAPI-4 DVD/CD]
+[...]
+5. Legacy option rom
+
+t. TPM Configuration
+
+
+The following commands should result in similar output inside the VM with a
+Linux kernel that either has the TPM TIS driver built-in or available as a
+module:
+
+#> dmesg | grep -i tpm
+[    0.711310] tpm_tis 00:06: 1.2 TPM (device=id 0x1, rev-id 1)
+
+#> dmesg | grep TCPA
+[    0.000000] ACPI: TCPA 0x0000000003FFD191C 000032 (v02 BOCHS  \
+    BXPCTCPA 0000001 BXPC 00000001)
+
+#> ls -l /dev/tpm*
+crw-------. 1 root root 10, 224 Jul 11 10:11 /dev/tpm0
+
+#> find /sys/devices/ | grep pcrs$ | xargs cat
+PCR-00: 35 4E 3B CE 23 9F 38 59 ...
+...
+PCR-23: 00 00 00 00 00 00 00 00 ...
diff --git a/hw/tpm/tpm_emulator.c b/hw/tpm/tpm_emulator.c
index 9aaec8e3efb79cea5c89b6c1c76ca1bda373fff0..e1a68104d6ff5e178c9545fe6dda4fe006d3608a 100644
--- a/hw/tpm/tpm_emulator.c
+++ b/hw/tpm/tpm_emulator.c
@@ -71,15 +71,21 @@ typedef struct TPMEmulator {
     ptm_cap caps; /* capabilities of the TPM */
     uint8_t cur_locty_number; /* last set locality */
     Error *migration_blocker;
+
+    QemuMutex mutex;
 } TPMEmulator;
 
 
-static int tpm_emulator_ctrlcmd(CharBackend *dev, unsigned long cmd, void *msg,
+static int tpm_emulator_ctrlcmd(TPMEmulator *tpm, unsigned long cmd, void *msg,
                                 size_t msg_len_in, size_t msg_len_out)
 {
+    CharBackend *dev = &tpm->ctrl_chr;
     uint32_t cmd_no = cpu_to_be32(cmd);
     ssize_t n = sizeof(uint32_t) + msg_len_in;
     uint8_t *buf = NULL;
+    int ret = -1;
+
+    qemu_mutex_lock(&tpm->mutex);
 
     buf = g_alloca(n);
     memcpy(buf, &cmd_no, sizeof(cmd_no));
@@ -87,17 +93,21 @@ static int tpm_emulator_ctrlcmd(CharBackend *dev, unsigned long cmd, void *msg,
 
     n = qemu_chr_fe_write_all(dev, buf, n);
     if (n <= 0) {
-        return -1;
+        goto end;
     }
 
     if (msg_len_out != 0) {
         n = qemu_chr_fe_read_all(dev, msg, msg_len_out);
         if (n <= 0) {
-            return -1;
+            goto end;
         }
     }
 
-    return 0;
+    ret = 0;
+
+end:
+    qemu_mutex_unlock(&tpm->mutex);
+    return ret;
 }
 
 static int tpm_emulator_unix_tx_bufs(TPMEmulator *tpm_emu,
@@ -154,7 +164,7 @@ static int tpm_emulator_set_locality(TPMEmulator *tpm_emu, uint8_t locty_number,
 
     DPRINTF("setting locality : 0x%x", locty_number);
     loc.u.req.loc = locty_number;
-    if (tpm_emulator_ctrlcmd(&tpm_emu->ctrl_chr, CMD_SET_LOCALITY, &loc,
+    if (tpm_emulator_ctrlcmd(tpm_emu, CMD_SET_LOCALITY, &loc,
                              sizeof(loc), sizeof(loc)) < 0) {
         error_setg(errp, "tpm-emulator: could not set locality : %s",
                    strerror(errno));
@@ -202,8 +212,8 @@ error:
 static int tpm_emulator_probe_caps(TPMEmulator *tpm_emu)
 {
     DPRINTF("%s", __func__);
-    if (tpm_emulator_ctrlcmd(&tpm_emu->ctrl_chr, CMD_GET_CAPABILITY,
-                         &tpm_emu->caps, 0, sizeof(tpm_emu->caps)) < 0) {
+    if (tpm_emulator_ctrlcmd(tpm_emu, CMD_GET_CAPABILITY,
+                             &tpm_emu->caps, 0, sizeof(tpm_emu->caps)) < 0) {
         error_report("tpm-emulator: probing failed : %s", strerror(errno));
         return -1;
     }
@@ -254,8 +264,8 @@ static int tpm_emulator_startup_tpm(TPMBackend *tb)
     ptm_res res;
 
     DPRINTF("%s", __func__);
-    if (tpm_emulator_ctrlcmd(&tpm_emu->ctrl_chr, CMD_INIT, &init, sizeof(init),
-                         sizeof(init)) < 0) {
+    if (tpm_emulator_ctrlcmd(tpm_emu, CMD_INIT, &init, sizeof(init),
+                             sizeof(init)) < 0) {
         error_report("tpm-emulator: could not send INIT: %s",
                      strerror(errno));
         goto err_exit;
@@ -278,7 +288,7 @@ static bool tpm_emulator_get_tpm_established_flag(TPMBackend *tb)
     ptm_est est;
 
     DPRINTF("%s", __func__);
-    if (tpm_emulator_ctrlcmd(&tpm_emu->ctrl_chr, CMD_GET_TPMESTABLISHED, &est,
+    if (tpm_emulator_ctrlcmd(tpm_emu, CMD_GET_TPMESTABLISHED, &est,
                              0, sizeof(est)) < 0) {
         error_report("tpm-emulator: Could not get the TPM established flag: %s",
                      strerror(errno));
@@ -302,7 +312,7 @@ static int tpm_emulator_reset_tpm_established_flag(TPMBackend *tb,
     }
 
     reset_est.u.req.loc = tpm_emu->cur_locty_number;
-    if (tpm_emulator_ctrlcmd(&tpm_emu->ctrl_chr, CMD_RESET_TPMESTABLISHED,
+    if (tpm_emulator_ctrlcmd(tpm_emu, CMD_RESET_TPMESTABLISHED,
                              &reset_est, sizeof(reset_est),
                              sizeof(reset_est)) < 0) {
         error_report("tpm-emulator: Could not reset the establishment bit: %s",
@@ -330,7 +340,7 @@ static void tpm_emulator_cancel_cmd(TPMBackend *tb)
         return;
     }
 
-    if (tpm_emulator_ctrlcmd(&tpm_emu->ctrl_chr, CMD_CANCEL_TPM_CMD, &res, 0,
+    if (tpm_emulator_ctrlcmd(tpm_emu, CMD_CANCEL_TPM_CMD, &res, 0,
                              sizeof(res)) < 0) {
         error_report("tpm-emulator: Could not cancel command: %s",
                      strerror(errno));
@@ -378,8 +388,8 @@ static int tpm_emulator_prepare_data_fd(TPMEmulator *tpm_emu)
 
     qemu_chr_fe_set_msgfds(&tpm_emu->ctrl_chr, fds + 1, 1);
 
-    if (tpm_emulator_ctrlcmd(&tpm_emu->ctrl_chr, CMD_SET_DATAFD, &res, 0,
-                    sizeof(res)) || res != 0) {
+    if (tpm_emulator_ctrlcmd(tpm_emu, CMD_SET_DATAFD, &res, 0,
+                             sizeof(res)) < 0 || res != 0) {
         error_report("tpm-emulator: Failed to send CMD_SET_DATAFD: %s",
                      strerror(errno));
         goto err_exit;
@@ -501,6 +511,7 @@ static void tpm_emulator_inst_init(Object *obj)
     DPRINTF("%s", __func__);
     tpm_emu->options = g_new0(TPMEmulatorOptions, 1);
     tpm_emu->cur_locty_number = ~0;
+    qemu_mutex_init(&tpm_emu->mutex);
 }
 
 /*
@@ -510,8 +521,7 @@ static void tpm_emulator_shutdown(TPMEmulator *tpm_emu)
 {
     ptm_res res;
 
-    if (tpm_emulator_ctrlcmd(&tpm_emu->ctrl_chr, CMD_SHUTDOWN, &res, 0,
-                             sizeof(res)) < 0) {
+    if (tpm_emulator_ctrlcmd(tpm_emu, CMD_SHUTDOWN, &res, 0, sizeof(res)) < 0) {
         error_report("tpm-emulator: Could not cleanly shutdown the TPM: %s",
                      strerror(errno));
     } else if (res != 0) {
@@ -536,6 +546,8 @@ static void tpm_emulator_inst_finalize(Object *obj)
         migrate_del_blocker(tpm_emu->migration_blocker);
         error_free(tpm_emu->migration_blocker);
     }
+
+    qemu_mutex_destroy(&tpm_emu->mutex);
 }
 
 static void tpm_emulator_class_init(ObjectClass *klass, void *data)
diff --git a/hw/tpm/tpm_tis.c b/hw/tpm/tpm_tis.c
index 7402528b256ce69d6125fa8082f985b351c7a734..42d647d363a9eedffa4efd061008bb9e0e01c134 100644
--- a/hw/tpm/tpm_tis.c
+++ b/hw/tpm/tpm_tis.c
@@ -545,7 +545,7 @@ static uint64_t tpm_tis_mmio_read(void *opaque, hwaddr addr,
     uint8_t v;
 
     if (tpm_backend_had_startup_error(s->be_driver)) {
-        return val;
+        return 0;
     }
 
     switch (offset) {
@@ -1008,6 +1008,10 @@ TPMVersion tpm_tis_get_tpm_version(Object *obj)
 {
     TPMState *s = TPM(obj);
 
+    if (tpm_backend_had_startup_error(s->be_driver)) {
+        return TPM_VERSION_UNSPEC;
+    }
+
     return tpm_backend_get_tpm_version(s->be_driver);
 }