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

Marc Zyngier
2026-04-30T10:37:11Z

On Mon, 27 Apr 2026 17:10:49 +0100,
Sascha Bischoff <Sascha.Bischoff@arm.com> 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 <sascha.bischoff@arm.com>
> ---
> 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.