On 22/01/25 01:22, Frederic Weisbecker wrote:
Le Tue, Jan 14, 2025 at 06:51:35PM +0100, Valentin Schneider a écrit :
ct_nmi_{enter, exit}() only touches the RCU watching counter and doesn't modify the actual CT state part context_tracking.state. This means that upon receiving an IRQ when idle, the CT_STATE_IDLE->CT_STATE_KERNEL transition only happens in ct_idle_exit().
One can note that ct_nmi_enter() can only ever be entered with the CT state as either CT_STATE_KERNEL or CT_STATE_IDLE, as an IRQ/NMI happenning in the CT_STATE_USER or CT_STATE_GUEST states will be routed down to ct_user_exit().
Are you sure? An NMI can fire between guest_state_enter_irqoff() and __svm_vcpu_run().
Urgh, you're quite right.
And NMIs interrupting userspace don't call enter_from_user_mode(). In fact they don't call irqentry_enter_from_user_mode() like regular IRQs but irqentry_nmi_enter() instead. Well that's for archs implementing common entry code, I can't speak for the others.
That I didn't realize, so thank you for pointing it out. Having another look now, I mistook DEFINE_IDTENTRY_RAW(exc_int3) for the general case when it really isn't :(
Unifying the behaviour between user and idle such that the IRQs/NMIs exit the CT_STATE can be interesting but I fear this may not come for free. You would need to save the old state on IRQ/NMI entry and restore it on exit.
That's what I tried to avoid, but it sounds like there's no nice way around it.
Do we really need it?
Well, my problem with not doing IDLE->KERNEL transitions on IRQ/NMI is that this leads the IPI deferral logic to observe a technically-out-of-sync sate for remote CPUs. Consider:
CPUx CPUy state := CT_STATE_IDLE ... ~>IRQ ... ct_nmi_enter() [in the kernel proper by now]
text_poke_bp_batch() ct_set_cpu_work(CPUy, CT_WORK_SYNC) READ CPUy ct->state `-> CT_IDLE_STATE `-> defer IPI
I thought this meant I would need to throw out the "defer IPIs if CPU is idle" part, but AIUI this also affects CT_STATE_USER and CT_STATE_GUEST, which is a bummer :(