Re: CPU Lockups in KVM with deferred hrtimer rearming

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 22, 2026 at 09:46:46AM +0200, Peter Zijlstra wrote:
> +		instrumentation_begin();
> +		/*
> +		 * KVM/VMX will dispatch from IRQ-disabled but for a context
> +		 * that will have IRQs-enabled. This confuses the entry code
> +		 * and it will not have reprogrammed the timer (or do
> +		 * preemption). Minimal fixup for now.
> +		 */
> +		hrtimer_rearm_deferred();
> +		instrumentation_end();

So I've been looking at this preemption thing. After having gotten my
head in a twist a few times around, I think this is done by
vcpu_enter_guest() like:

	preempt_disable();
	local_irq_disable();
	...
	kvm_x86_call(handle_exit_irqoff)(vcpu); <--- all of this VMX nonsense
	...
	local_irq_enable();
				<--- WTF goes here :-)
	++vcpu->stat.exits;
	local_irq_disable();
	...
	local_irq_enable();
	preempt_enable(); <--- here we finally preempt

This earlier IRQ-enable makes my head hurt and I had to go buy a new
WTF'o'meter (again!).