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

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Message-ID: <20211220161014.GC918551@lothringen>
Date:   Mon, 20 Dec 2021 17:10:14 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Nicolas Saenz Julienne <nsaenzju@...hat.com>, maz <maz@...nel.org>,
        Will Deacon <will@...nel.org>, paulmck <paulmck@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        rcu <rcu@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
        kvmarm@...ts.cs.columbia.edu,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: 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:
> > 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()?

Thanks.