um: time-travel: support time-travel protocol broadcast messages

Add a message type to the time-travel protocol to broadcast
a small (64-bit) value to all participants in a simulation.
The main use case is to have an identical message come to
all participants in a simulation, e.g. to separate out logs
for different tests running in a single simulation.

Down in the guts of time_travel_handle_message() we can't
use printk() and not even printk_deferred(), so just store
the message and print it at the start of the userspace()
function.

Unfortunately this means that other prints in the kernel
can actually bypass the message, but in most cases where
this is used, for example to separate test logs, userspace
will be involved. Also, even if we could use
printk_deferred(), we'd still need to flush it out in the
userspace() function since otherwise userspace messages
might cross it.

As a result, this is a reasonable compromise, there's no
need to have any core changes and it solves the main use
case we have for it.

Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Link: https://patch.msgid.link/20240702192118.c4093bc5b15e.I2ca8d006b67feeb866ac2017af7b741c9e06445a@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

arch/um/include/shared/timetravel.h

@@ -15,8 +15,17 @@ enum time_travel_mode {
 #if defined(UML_CONFIG_UML_TIME_TRAVEL_SUPPORT) || \
     defined(CONFIG_UML_TIME_TRAVEL_SUPPORT)
 extern enum time_travel_mode time_travel_mode;
+extern int time_travel_should_print_bc_msg;
 #else
 #define time_travel_mode TT_MODE_OFF
+#define time_travel_should_print_bc_msg 0
 #endif /* (UML_)CONFIG_UML_TIME_TRAVEL_SUPPORT */
 
+void _time_travel_print_bc_msg(void);
+static inline void time_travel_print_bc_msg(void)
+{
+	if (time_travel_should_print_bc_msg)
+		_time_travel_print_bc_msg();
+}
+
 #endif /* _UM_TIME_TRAVEL_H_ */

arch/um/kernel/time.c

@@ -60,6 +60,15 @@ enum time_travel_message_handling {
 	TTMH_READ,
 };
 
+static u64 bc_message;
+int time_travel_should_print_bc_msg;
+
+void _time_travel_print_bc_msg(void)
+{
+	time_travel_should_print_bc_msg = 0;
+	printk(KERN_INFO "time-travel: received broadcast 0x%llx\n", bc_message);
+}
+
 static void time_travel_handle_message(struct um_timetravel_msg *msg,
 				       enum time_travel_message_handling mode)
 {
@@ -101,6 +110,10 @@ static void time_travel_handle_message(struct um_timetravel_msg *msg,
 		time_travel_ext_free_until_valid = true;
 		time_travel_ext_free_until = msg->time;
 		break;
+	case UM_TIMETRAVEL_BROADCAST:
+		bc_message = msg->time;
+		time_travel_should_print_bc_msg = 1;
+		break;
 	}
 
 	resp.seq = msg->seq;
@@ -880,4 +893,50 @@ __uml_help(setup_time_travel_start,
 "time-travel-start=<nanoseconds>\n"
 "Configure the UML instance's wall clock to start at this value rather than\n"
 "the host's wall clock at the time of UML boot.\n");
+static struct kobject *bc_time_kobject;
+
+static ssize_t bc_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "0x%llx", bc_message);
+}
+
+static ssize_t bc_store(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t count)
+{
+	int ret;
+	u64 user_bc_message;
+
+	ret = kstrtou64(buf, 0, &user_bc_message);
+	if (ret)
+		return ret;
+
+	bc_message = user_bc_message;
+
+	time_travel_ext_req(UM_TIMETRAVEL_BROADCAST, bc_message);
+	pr_info("um: time: sent broadcast message: 0x%llx\n", bc_message);
+	return count;
+}
+
+static struct kobj_attribute bc_attribute = __ATTR(bc-message, 0660, bc_show, bc_store);
+
+static int __init um_bc_start(void)
+{
+	if (time_travel_mode != TT_MODE_EXTERNAL)
+		return 0;
+
+	bc_time_kobject = kobject_create_and_add("um-ext-time", kernel_kobj);
+	if (!bc_time_kobject)
+		return 0;
+
+	if (sysfs_create_file(bc_time_kobject, &bc_attribute.attr))
+		pr_debug("failed to create the bc file in /sys/kernel/um_time");
+
+	return 0;
+}
+
+void __exit time_exit(void)
+{
+	kobject_put(bc_time_kobject);
+}
+
+late_initcall(um_bc_start);
 #endif

arch/um/os-Linux/skas/process.c

@@ -23,6 +23,7 @@
 #include <skas.h>
 #include <sysdep/stub.h>
 #include <linux/threads.h>
+#include <timetravel.h>
 #include "../internal.h"
 
 int is_skas_winch(int pid, int fd, void *data)
@@ -345,6 +346,8 @@ void userspace(struct uml_pt_regs *regs, unsigned long *aux_fp_regs)
 	interrupt_end();
 
 	while (1) {
+		time_travel_print_bc_msg();
+
 		if (kill_userspace_mm[0])
 			fatal_sigsegv();
 

include/uapi/linux/um_timetravel.h

@@ -123,6 +123,17 @@ enum um_timetravel_ops {
 	 *	the simulation.
 	 */
 	UM_TIMETRAVEL_GET_TOD		= 8,
+
+	/**
+	 * @UM_TIMETRAVEL_BROADCAST: Send/Receive a broadcast message.
+	 *	This message can be used to sync all components in the system
+	 *	with a single message, if the calender gets the message, the
+	 *	calender broadcast the message to all components, and if a
+	 *	component receives it it should act based on it e.g print a
+	 *	message to it's log system.
+	 *	(calendar <-> host)
+	 */
+	UM_TIMETRAVEL_BROADCAST		= 9,
 };
 
 #endif /* _UAPI_LINUX_UM_TIMETRAVEL_H */