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, 25 insertions, 14 deletions

diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index 820846ff7179..d6c22f8aa06d 100644
--- a/arch/um/os-Linux/skas/process.c
+++ b/arch/um/os-Linux/skas/process.c
@@ -546,7 +546,7 @@ extern unsigned long tt_extra_sched_jiffies;
 void userspace(struct uml_pt_regs *regs)
 {
 	int err, status, op;
-	siginfo_t si_ptrace;
+	siginfo_t si_local;
 	siginfo_t *si;
 	int sig;
 
@@ -557,6 +557,13 @@ void userspace(struct uml_pt_regs *regs)
 		struct mm_id *mm_id = current_mm_id();
 
 		/*
+		 * At any given time, only one CPU thread can enter the
+		 * turnstile to operate on the same stub process, including
+		 * executing stub system calls (mmap and munmap).
+		 */
+		enter_turnstile(mm_id);
+
+		/*
 		 * When we are in time-travel mode, userspace can theoretically
 		 * do a *lot* of work without being scheduled. The problem with
 		 * this is that it will prevent kernel bookkeeping (primarily
@@ -623,9 +630,10 @@ void userspace(struct uml_pt_regs *regs)
 			}
 
 			if (proc_data->si_offset > sizeof(proc_data->sigstack) - sizeof(*si))
-				panic("%s - Invalid siginfo offset from child",
-				      __func__);
-			si = (void *)&proc_data->sigstack[proc_data->si_offset];
+				panic("%s - Invalid siginfo offset from child", __func__);
+
+			si = &si_local;
+			memcpy(si, &proc_data->sigstack[proc_data->si_offset], sizeof(*si));
 
 			regs->is_user = 1;
 
@@ -721,8 +729,8 @@ void userspace(struct uml_pt_regs *regs)
 				case SIGFPE:
 				case SIGWINCH:
 					ptrace(PTRACE_GETSIGINFO, pid, 0,
-					       (struct siginfo *)&si_ptrace);
-					si = &si_ptrace;
+					       (struct siginfo *)&si_local);
+					si = &si_local;
 					break;
 				default:
 					si = NULL;
@@ -733,6 +741,8 @@ void userspace(struct uml_pt_regs *regs)
 			}
 		}
 
+		exit_turnstile(mm_id);
+
 		UPT_SYSCALL_NR(regs) = -1; /* Assume: It's not a syscall */
 
 		if (sig) {
@@ -802,10 +812,9 @@ void switch_threads(jmp_buf *me, jmp_buf *you)
 
 static jmp_buf initial_jmpbuf;
 
-/* XXX Make these percpu */
-static void (*cb_proc)(void *arg);
-static void *cb_arg;
-static jmp_buf *cb_back;
+static __thread void (*cb_proc)(void *arg);
+static __thread void *cb_arg;
+static __thread jmp_buf *cb_back;
 
 int start_idle_thread(void *stack, jmp_buf *switch_buf)
 {
@@ -859,10 +868,10 @@ void initial_thread_cb_skas(void (*proc)(void *), void *arg)
 	cb_arg = arg;
 	cb_back = &here;
 
-	block_signals_trace();
+	initial_jmpbuf_lock();
 	if (UML_SETJMP(&here) == 0)
 		UML_LONGJMP(&initial_jmpbuf, INIT_JMP_CALLBACK);
-	unblock_signals_trace();
+	initial_jmpbuf_unlock();
 
 	cb_proc = NULL;
 	cb_arg = NULL;
@@ -871,8 +880,9 @@ void initial_thread_cb_skas(void (*proc)(void *), void *arg)
 
 void halt_skas(void)
 {
-	block_signals_trace();
+	initial_jmpbuf_lock();
 	UML_LONGJMP(&initial_jmpbuf, INIT_JMP_HALT);
+	/* unreachable */
 }
 
 static bool noreboot;
@@ -892,6 +902,7 @@ __uml_setup("noreboot", noreboot_cmd_param,
 
 void reboot_skas(void)
 {
-	block_signals_trace();
+	initial_jmpbuf_lock();
 	UML_LONGJMP(&initial_jmpbuf, noreboot ? INIT_JMP_HALT : INIT_JMP_REBOOT);
+	/* unreachable */
 }