Philippe Mathieu-Daudé <phi...@linaro.org> writes: > Hi Alex, > > On 29/5/24 17:22, Alex Bennée wrote: >> This ensures we don't start the thread until cpu_common_realizefn has >> finished. This ensures that plugins will always run >> qemu_plugin_vcpu_init__async first before any other states. It doesn't >> totally eliminate the race that plugin_cpu_update__locked has to work >> around though. I found this while reviewing the ips plugin which makes >> heavy use of the vcpu phase callbacks. >> An alternative might be to move the explicit creation of vCPU >> threads >> to qdev_machine_creation_done()? It doesn't affect user-mode which >> already has a thread to execute in and ensures the QOM object has >> completed creation in cpu_create() before continuing. >> Signed-off-by: Alex Bennée <alex.ben...@linaro.org> >> Cc: Pierrick Bouvier <pierrick.bouv...@linaro.org> >> Cc: Philippe Mathieu-Daudé <phi...@linaro.org> >> --- >> include/hw/core/cpu.h | 8 ++++++++ >> accel/tcg/user-exec-stub.c | 5 +++++ >> hw/core/cpu-common.c | 7 ++++++- >> plugins/core.c | 5 +++++ >> system/cpus.c | 15 ++++++++++----- >> 5 files changed, 34 insertions(+), 6 deletions(-) >> diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h >> index bb398e8237..6920699585 100644 >> --- a/include/hw/core/cpu.h >> +++ b/include/hw/core/cpu.h >> @@ -1041,6 +1041,14 @@ void end_exclusive(void); >> */ >> void qemu_init_vcpu(CPUState *cpu); >> +/** >> + * qemu_start_vcpu: >> + * @cpu: The vCPU to start. >> + * >> + * Create the vCPU thread and start it running. >> + */ >> +void qemu_start_vcpu(CPUState *cpu); >> + >> #define SSTEP_ENABLE 0x1 /* Enable simulated HW single stepping */ >> #define SSTEP_NOIRQ 0x2 /* Do not use IRQ while single stepping */ >> #define SSTEP_NOTIMER 0x4 /* Do not Timers while single stepping */ >> diff --git a/accel/tcg/user-exec-stub.c b/accel/tcg/user-exec-stub.c >> index 4fbe2dbdc8..162bb72bbe 100644 >> --- a/accel/tcg/user-exec-stub.c >> +++ b/accel/tcg/user-exec-stub.c >> @@ -18,6 +18,11 @@ void cpu_exec_reset_hold(CPUState *cpu) >> { >> } >> +void qemu_start_vcpu(CPUState *cpu) >> +{ >> + /* NOP for user-mode, we already have a thread */ >> +} >> + >> /* User mode emulation does not support record/replay yet. */ >> bool replay_exception(void) >> diff --git a/hw/core/cpu-common.c b/hw/core/cpu-common.c >> index 0f0a247f56..68895ddd59 100644 >> --- a/hw/core/cpu-common.c >> +++ b/hw/core/cpu-common.c >> @@ -230,7 +230,12 @@ static void cpu_common_realizefn(DeviceState *dev, >> Error **errp) >> } >> #endif >> - /* NOTE: latest generic point where the cpu is fully realized >> */ >> + /* >> + * With everything set up we can finally start the vCPU thread. >> + * This is a NOP for linux-user. >> + * NOTE: latest generic point where the cpu is fully realized >> + */ >> + qemu_start_vcpu(cpu); >> } >> static void cpu_common_unrealizefn(DeviceState *dev) >> diff --git a/plugins/core.c b/plugins/core.c >> index 0726bc7f25..1e5da7853b 100644 >> --- a/plugins/core.c >> +++ b/plugins/core.c >> @@ -65,6 +65,11 @@ static void plugin_cpu_update__locked(gpointer k, >> gpointer v, gpointer udata) >> CPUState *cpu = container_of(k, CPUState, cpu_index); >> run_on_cpu_data mask = RUN_ON_CPU_HOST_ULONG(*plugin.mask); >> + /* >> + * There is a race condition between the starting of the vCPU >> + * thread at the end of cpu_common_realizefn and when realized is >> + * finally set. >> + */ > > I'd like we simply assert(DEVICE(cpu)->realized) here; > I still don't understand when this can be called while > the vcpu isn't yet realized.
It will be shortly but as the comment says we don't set it until we come out of cpu_common_realizefn and return to QOM so you get: ** ERROR:../../plugins/core.c:73:plugin_cpu_update__locked: assertion failed: (DEVICE(cpu)->realized) Bail out! ERROR:../../plugins/core.c:73:plugin_cpu_update__locked: assertion failed: (DEVICE(cpu)->realized) Thread 4 "qemu-system-aar" received signal SIGABRT, Aborted. [Switching to Thread 0x7fffe5e006c0 (LWP 1000969)] __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at ./nptl/pthread_kill.c:44 44 ./nptl/pthread_kill.c: No such file or directory. (gdb) bt #0 __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at ./nptl/pthread_kill.c:44 #1 0x00007ffff4ca9e8f in __pthread_kill_internal (signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:78 #2 0x00007ffff4c5afb2 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26 #3 0x00007ffff4c45472 in __GI_abort () at ./stdlib/abort.c:79 #4 0x00007ffff6e46ec8 in () at /lib/x86_64-linux-gnu/libglib-2.0.so.0 #5 0x00007ffff6ea6e1a in g_assertion_message_expr () at /lib/x86_64-linux-gnu/libglib-2.0.so.0 #6 0x000055555600e2c8 in plugin_cpu_update__locked (k=0x5555579e9ee8, v=<optimized out>, udata=<optimized out>) at ../../plugins/core.c:73 #7 0x000055555600e5fc in qemu_plugin_vcpu_init_hook (cpu=0x5555579e9c20) at ../../plugins/core.c:261 #8 0x00005555558ff106 in process_queued_cpu_work (cpu=0x5555579e9c20) at ../../cpu-common.c:360 #9 0x00005555560100f6 in mttcg_cpu_thread_fn (arg=arg@entry=0x5555579e9c20) at ../../accel/tcg/tcg-accel-ops-mttcg.c:118 #10 0x00005555561bcce8 in qemu_thread_start (args=0x555557a55d70) at ../../util/qemu-thread-posix.c:541 #11 0x00007ffff4ca8134 in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:442 #12 0x00007ffff4d287dc in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81 Arguably I think we should just wait until machine is created I think. > >> if (DEVICE(cpu)->realized) { >> async_run_on_cpu(cpu, plugin_cpu_update__async, mask); >> } else { >> diff --git a/system/cpus.c b/system/cpus.c >> index d3640c9503..7dd8464c5e 100644 >> --- a/system/cpus.c >> +++ b/system/cpus.c >> @@ -488,11 +488,13 @@ void cpus_kick_thread(CPUState *cpu) >> void qemu_cpu_kick(CPUState *cpu) >> { >> - qemu_cond_broadcast(cpu->halt_cond); >> - if (cpus_accel->kick_vcpu_thread) { >> - cpus_accel->kick_vcpu_thread(cpu); >> - } else { /* default */ >> - cpus_kick_thread(cpu); >> + if (cpu->halt_cond) { > > cpu->halt_cond = NULL is a bug, why kicking a vcpu not > yet fully created? We are queuing work for when it is ready but we don't create cpu->halt_cond until in the thread (this is mainly to workaround the fact the rr_thread shares its context between multiple CPUStates). > >> + qemu_cond_broadcast(cpu->halt_cond); >> + if (cpus_accel->kick_vcpu_thread) { >> + cpus_accel->kick_vcpu_thread(cpu); >> + } else { /* default */ >> + cpus_kick_thread(cpu); >> + } >> } >> } >> @@ -674,7 +676,10 @@ void qemu_init_vcpu(CPUState *cpu) >> cpu->num_ases = 1; >> cpu_address_space_init(cpu, 0, "cpu-memory", cpu->memory); >> } >> +} >> +void qemu_start_vcpu(CPUState *cpu) >> +{ >> /* accelerators all implement the AccelOpsClass */ >> g_assert(cpus_accel != NULL && cpus_accel->create_vcpu_thread != NULL); >> cpus_accel->create_vcpu_thread(cpu); -- Alex Bennée Virtualisation Tech Lead @ Linaro