Subject: Re: [PATCH 14/43] KVM: arm64: gic-v5: Request VPE doorbells when going non-resident

From: Marc Zyngier <[email-masked]>
Date: Thu, 30 Apr 2026 11:37:08 +0100

On Mon, 27 Apr 2026 17:10:49 +0100,
Sascha Bischoff <[email-masked]> wrote:
> 
> When a VPE is made non-resident and is entering WFI, a doorbell should
> be requested for the VPE. This allows the VPE to be easily woken once
> an SPI/LPI interrupt is pending for it. This is tracked by the IRS,
> which will signal the specific VPE doorbell for the VPE once such an
> interrupt arrives.
> 
> Requesting a doorbell involves calculating the DBPM - DoorBell
> Priority Mask - which ensures that the DB is only signalled by the
> hardware if the pending interrupt is of sufficient priority. This
> avoids waking a VPE that can't process the incoming interrupt.
> 
> Doorbells are NOT requested if a VPE is not entering WFI as we expect
> to enter again imminently.
> 
> Signed-off-by: Sascha Bischoff <[email-masked]>
> ---
>  arch/arm64/kvm/vgic/vgic-v5.c | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/arch/arm64/kvm/vgic/vgic-v5.c b/arch/arm64/kvm/vgic/vgic-v5.c
> index 11a1a491b7e0a..2fc6fa4df034f 100644
> --- a/arch/arm64/kvm/vgic/vgic-v5.c
> +++ b/arch/arm64/kvm/vgic/vgic-v5.c
> @@ -1077,6 +1077,9 @@ void vgic_v5_load(struct kvm_vcpu *vcpu)
>  void vgic_v5_put(struct kvm_vcpu *vcpu)
>  {
>  	struct vgic_v5_cpu_if *cpu_if = &vcpu->arch.vgic_cpu.vgic_v5;
> +	bool req_db = !!vcpu_get_flag(vcpu, IN_WFI);

Drop the spurious variable and move the check in the if () statement.
This is way more readable than declaring a variable.

> +	u32 priority_mask;
> +	int dbpm;

Move these in the inner block.

>  
>  	/*
>  	 * Do nothing if we're not resident. This can happen in the WFI path
> @@ -1090,6 +1093,31 @@ void vgic_v5_put(struct kvm_vcpu *vcpu)
>  	kvm_call_hyp(__vgic_v5_save_apr, cpu_if);
>  
>  	cpu_if->vgic_contextr = 0;
> +	if (req_db) {
> +		/*
> +		 * Find the virtual running priority and use this to calculate
> +		 * the doorbell priority mask. We combine the highest active
> +		 * priority and the CPU's priority mask. The guest can't handle
> +		 * interrupts with priorities less than or equal to the virtual
> +		 * running priority, so there's literally no point in waking the
> +		 * guest for these.
> +		 *
> +		 * The priority needs to be higher than the mask to signal, so
> +		 * pick the next higher priority (subtract 1).
> +		 */
> +		priority_mask = vgic_v5_get_effective_priority_mask(vcpu);
> +
> +		/* Don't request a doorbell if the max priority is masked */

This comment reads badly. I'd suggest something like "Request a
doorbell *unless* the priority is 0, indicating that no interrupt can
wake the vcpu up".

> +		if (priority_mask) {
> +			dbpm = priority_mask - 1;
> +			cpu_if->vgic_contextr = FIELD_PREP(ICH_CONTEXTR_EL2_DB, 1) |
> +						FIELD_PREP(ICH_CONTEXTR_EL2_DBPM, dbpm);
> +		}
> +
> +		/* Make the doorbell affine to this CPU */
> +		WARN_ON(irq_set_affinity(vgic_v5_vpe_db(vcpu),
> +					 cpumask_of(smp_processor_id())));

Repeatedly setting the affinity is likely to be costly. It may be
worth comparing with the current affinity somehow.

> +	}
>  
>  	kvm_call_hyp(__vgic_v5_make_non_resident, cpu_if);
>  

Thanks,

	M.

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