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

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Dec 20, 2021 at 05:10:14PM +0100, Frederic Weisbecker wrote:
> 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:
> > > Hi All,
> > 
> > Hi,
> > 
> > > 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()?

Due to lockep/tracing/etc ordering, we don't use irq_enter() directly and
instead call rcu_irq_enter() and irq_enter_rcu() separately. Critically we only
call rcu_irq_enter() for IRQs taken from the idle thread, as this was
previously thought to be the only place where we could take an IRQ from an EL1
EQS.

See __el1_irq(), __enter_from_kernel_mode(), and __exit_to_kernel_mode() in
arch/arm64/kernel/entry-common.c. The latter two are largely analogous to the
common irqentry_enter9) and irqentry_exit() helpers in kernel/entry/common.c.

We need to either rework the KVM code or that entry code. I'll dig into this a
bit more...

Thanks,
Mark.