[PATCH] KVM: arm64: Wake-up from WFI when iqrchip is in userspace

[Yao Yuan]

On Fri, Apr 24, 2026 at 08:24:42AM +0800, Marc Zyngier wrote:
> On Fri, 24 Apr 2026 07:33:02 +0100,
> Yao Yuan <yaoyuan at linux.alibaba.com> wrote:
> >
> > On Thu, Apr 23, 2026 at 05:36:07PM +0800, Marc Zyngier wrote:
> > > It appears that there is nothing in the wake-up path that
> > > evaluates whether the in-kernel interrupts are pending unless
> > > we have a vgic.
> > >
> > > This means that the userspace irqchip support has been broken for
> > > about four years, and nobody noticed. It was also broken before
> > > as we wouldn't wake-up on a PMU interrupt, but hey, who cares...
> > >
> > > It is probably time to remove the feature altogether, because it
> > > was a terrible idea 10 years ago, and it still is.
> > >
> > > Fixes: b57de4ffd7c6d ("KVM: arm64: Simplify kvm_cpu_has_pending_timer()")
> > > Signed-off-by: Marc Zyngier <maz at kernel.org>
> > > ---
> > >  arch/arm64/kvm/arm.c | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > >
> > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > > index 176cbe8baad30..8bb2c7422cc8b 100644
> > > --- a/arch/arm64/kvm/arm.c
> > > +++ b/arch/arm64/kvm/arm.c
> > > @@ -824,6 +824,10 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *v)
> > >  {
> > >  	bool irq_lines = *vcpu_hcr(v) & (HCR_VI | HCR_VF | HCR_VSE);
> > >
> >
> > Hi Marc,
> >
> > > +	irq_lines |= (!irqchip_in_kernel(v->kvm) &&
> > > +		      (kvm_timer_should_notify_user(v) ||
> > > +		       kvm_pmu_should_notify_user(v)));
> >
> > How about a new helper like 'kvm_should_notify_us_irqchip()' ?
> > We can replace the same part at beginning of kvm_vcpu_exit_request() and
> > here w/ unlikely().
>
> I'd rather not introduce a helper, for two reasons:
>
> - this needs to be backported all the way to 5.19, because that's how
>   far it has been broken. So keeping it small and localised is far
>   better than introducing a helper that will make the backport less
>   obvious.

Agree on this point very much!

>
> - I have patches to remove the other calls to kvm_*_notify_user() as a
>   simplification of this utterly stupid feature.

OK.

>
> Finally, and while I agree that this could take an unlikely()
> qualifier, a much better course of action would be to have a separate
> patch that moves the qualifier to the predicate itself.

I got it, thanks for your such detail explanation!

>
> Thanks,
>
> 	M.
>
> --
> Jazz isn't dead. It just smells funny.