um: Add initial SMP support

Add initial symmetric multi-processing (SMP) support to UML. With
this support enabled, users can tell UML to start multiple virtual
processors, each represented as a separate host thread.

In UML, kthreads and normal threads (when running in kernel mode)
can be scheduled and executed simultaneously on different virtual
processors. However, the userspace code of normal threads still
runs within their respective single-threaded stubs.

That is, SMP support is currently available both within the kernel
and across different processes, but still remains limited within
threads of the same process in userspace.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20251027001815.1666872-6-tiwei.bie@linux.dev
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

1 files changed, 148 insertions, 0 deletions

diff --git a/arch/um/os-Linux/smp.c b/arch/um/os-Linux/smp.c
new file mode 100644
index 000000000000..18d3858a7cd2
--- /dev/null
+++ b/arch/um/os-Linux/smp.c
@@ -0,0 +1,148 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2025 Ant Group
+ * Author: Tiwei Bie <tiwei.btw@antgroup.com>
+ */
+
+#include <errno.h>
+#include <pthread.h>
+#include <signal.h>
+#include <kern_util.h>
+#include <um_malloc.h>
+#include <init.h>
+#include <os.h>
+#include <smp.h>
+#include "internal.h"
+
+struct cpu_thread_data {
+	int cpu;
+	sigset_t sigset;
+};
+
+static __thread int __curr_cpu;
+
+int uml_curr_cpu(void)
+{
+	return __curr_cpu;
+}
+
+static pthread_t cpu_threads[CONFIG_NR_CPUS];
+
+static void *cpu_thread(void *arg)
+{
+	struct cpu_thread_data *data = arg;
+
+	__curr_cpu = data->cpu;
+
+	uml_start_secondary(data);
+
+	return NULL;
+}
+
+int os_start_cpu_thread(int cpu)
+{
+	struct cpu_thread_data *data;
+	sigset_t sigset, oset;
+	int err;
+
+	data = uml_kmalloc(sizeof(*data), UM_GFP_ATOMIC);
+	if (!data)
+		return -ENOMEM;
+
+	sigfillset(&sigset);
+	if (sigprocmask(SIG_SETMASK, &sigset, &oset) < 0) {
+		err = errno;
+		goto err;
+	}
+
+	data->cpu = cpu;
+	data->sigset = oset;
+
+	err = pthread_create(&cpu_threads[cpu], NULL, cpu_thread, data);
+	if (sigprocmask(SIG_SETMASK, &oset, NULL) < 0)
+		panic("Failed to restore the signal mask, errno = %d", errno);
+	if (err != 0)
+		goto err;
+
+	return 0;
+
+err:
+	kfree(data);
+	return -err;
+}
+
+void os_start_secondary(void *arg, jmp_buf *switch_buf)
+{
+	struct cpu_thread_data *data = arg;
+
+	sigaddset(&data->sigset, IPI_SIGNAL);
+	sigaddset(&data->sigset, SIGIO);
+
+	if (sigprocmask(SIG_SETMASK, &data->sigset, NULL) < 0)
+		panic("Failed to restore the signal mask, errno = %d", errno);
+
+	kfree(data);
+	longjmp(*switch_buf, 1);
+
+	/* unreachable */
+	printk(UM_KERN_ERR "impossible long jump!");
+	fatal_sigsegv();
+}
+
+int os_send_ipi(int cpu, int vector)
+{
+	union sigval value = { .sival_int = vector };
+
+	return pthread_sigqueue(cpu_threads[cpu], IPI_SIGNAL, value);
+}
+
+static void __local_ipi_set(int enable)
+{
+	sigset_t sigset;
+
+	sigemptyset(&sigset);
+	sigaddset(&sigset, IPI_SIGNAL);
+
+	if (sigprocmask(enable ? SIG_UNBLOCK : SIG_BLOCK, &sigset, NULL) < 0)
+		panic("%s: sigprocmask failed, errno = %d", __func__, errno);
+}
+
+void os_local_ipi_enable(void)
+{
+	__local_ipi_set(1);
+}
+
+void os_local_ipi_disable(void)
+{
+	__local_ipi_set(0);
+}
+
+static void ipi_sig_handler(int sig, siginfo_t *si, void *uc)
+{
+	int save_errno = errno;
+
+	signals_enabled = 0;
+	um_trace_signals_off();
+
+	uml_ipi_handler(si->si_value.sival_int);
+
+	um_trace_signals_on();
+	signals_enabled = 1;
+
+	errno = save_errno;
+}
+
+void __init os_init_smp(void)
+{
+	struct sigaction action = {
+		.sa_sigaction = ipi_sig_handler,
+		.sa_flags = SA_SIGINFO | SA_ONSTACK | SA_RESTART,
+	};
+
+	sigfillset(&action.sa_mask);
+
+	if (sigaction(IPI_SIGNAL, &action, NULL) < 0)
+		panic("%s: sigaction failed, errno = %d", __func__, errno);
+
+	cpu_threads[0] = pthread_self();
+}