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

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

[Marc Zyngier]

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@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);
 
+	irq_lines |= (!irqchip_in_kernel(v->kvm) &&
+		      (kvm_timer_should_notify_user(v) ||
+		       kvm_pmu_should_notify_user(v)));
+
 	return ((irq_lines || kvm_vgic_vcpu_pending_irq(v))
 		&& !kvm_arm_vcpu_stopped(v) && !v->arch.pause);
 }
-- 
2.47.3

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

[Yao Yuan]

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@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().

> +
>  	return ((irq_lines || kvm_vgic_vcpu_pending_irq(v))
>  		&& !kvm_arm_vcpu_stopped(v) && !v->arch.pause);
>  }
> --
> 2.47.3
>

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

[Marc Zyngier]

On Fri, 24 Apr 2026 07:33:02 +0100,
Yao Yuan <yaoyuan@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@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.

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

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.

Thanks,

	M.

-- 
Jazz isn't dead. It just smells funny.

Re: [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@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@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.

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

[Marc Zyngier]

On Thu, 23 Apr 2026 17:36:07 +0100, 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...
> 
> [...]

Applied to fixes, thanks!

[1/1] KVM: arm64: Wake-up from WFI when iqrchip is in userspace
      commit: 4ce98bf0865c349e7026ad9c14f48da264920953

Cheers,

	M.
-- 
Without deviation from the norm, progress is not possible.