Re: Possible nohz-full/RCU issue in arm64 KVM

On Fri, Dec 17, 2021 at 01:21:39PM +0000, Mark Rutland wrote:

On Fri, Dec 17, 2021 at 12:51:57PM +0100, Nicolas Saenz Julienne wrote:

quoted

Hi All,

Hi,

quoted

arm64's guest entry code does the following:

int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
{
	[...]

	guest_enter_irqoff();

	ret = kvm_call_hyp_ret(__kvm_vcpu_run, vcpu);

	[...]

	local_irq_enable();

	/*
	 * We do local_irq_enable() before calling guest_exit() so
	 * that if a timer interrupt hits while running the guest we
	 * account that tick as being spent in the guest.  We enable
	 * preemption after calling guest_exit() so that if we get
	 * preempted we make sure ticks after that is not counted as
	 * guest time.
	 */
	guest_exit();
	[...]
}


On a nohz-full CPU, guest_{enter,exit}() delimit an RCU extended quiescent
state (EQS). Any interrupt happening between local_irq_enable() and
guest_exit() should disable that EQS. Now, AFAICT all el0 interrupt handlers
do the right thing if trggered in this context, but el1's won't. Is it
possible to hit an el1 handler (for example __el1_irq()) there?

I think you're right that the EL1 handlers can trigger here and won't exit the
EQS.

I'm not immediately sure what we *should* do here. What does x86 do for an IRQ
taken from a guest mode? I couldn't spot any handling of that case, but I'm not
familiar enough with the x86 exception model to know if I'm looking in the
right place.

This is one of the purposes of rcu_irq_enter(). el1 handlers don't call irq_enter()?

Thanks.

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