Subject: Re: [PATCH v4 10/36] KVM: arm64: gic-v5: Detect implemented PPIs on boot

From: Marc Zyngier <[email-masked]>
Date: Fri, 30 Jan 2026 11:03:43 +0000

On Wed, 28 Jan 2026 18:01:54 +0000,
Sascha Bischoff <[email-masked]> wrote:
> 
> As part of booting the system and initialising KVM, create and
> populate a mask of the implemented PPIs. This mask allows future PPI
> operations (such as save/restore or state, or syncing back into the
> shadow state) to only consider PPIs that are actually implemented on
> the host.
> 
> The set of implemented virtual PPIs matches the set of implemented
> physical PPIs for a GICv5 host. Therefore, this mask represents all
> PPIs that could ever by used by a GICv5-based guest on a specific
> host.
> 
> Only architected PPIs are currently supported in KVM with
> GICv5. Moreover, as KVM only supports a subset of all possible PPIS
> (Timers, PMU, GICv5 SW_PPI) the PPI mask only includes these PPIs, if
> present. The timers are always assumed to be present; if we have KVM
> we have EL2, which means that we have the EL1 & EL2 Timer PPIs. If we
> have a PMU (v3), then the PMUIRQ is present. The GICv5 SW_PPI is
> always assumed to be present.
> 
> Signed-off-by: Sascha Bischoff <[email-masked]>
> ---
>  arch/arm64/kvm/vgic/vgic-init.c    |  4 ++++
>  arch/arm64/kvm/vgic/vgic-v5.c      | 33 ++++++++++++++++++++++++++++++
>  arch/arm64/kvm/vgic/vgic.h         |  1 +
>  include/kvm/arm_vgic.h             |  5 +++++
>  include/linux/irqchip/arm-gic-v5.h | 10 +++++++++
>  5 files changed, 53 insertions(+)
> 
> diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
> index 86c149537493..653364299154 100644
> --- a/arch/arm64/kvm/vgic/vgic-init.c
> +++ b/arch/arm64/kvm/vgic/vgic-init.c
> @@ -750,5 +750,9 @@ int kvm_vgic_hyp_init(void)
>  	}
>  
>  	kvm_info("vgic interrupt IRQ%d\n", kvm_vgic_global_state.maint_irq);
> +
> +	/* Always safe to call */
> +	vgic_v5_get_implemented_ppis();

What is the reason for calling this from the generic code, while it is
v5-specific? I'd have expected this to be entirely contained in the v5
subsystem.

> +
>  	return 0;
>  }
> diff --git a/arch/arm64/kvm/vgic/vgic-v5.c b/arch/arm64/kvm/vgic/vgic-v5.c
> index 23d0a495d855..9bd5a85ba203 100644
> --- a/arch/arm64/kvm/vgic/vgic-v5.c
> +++ b/arch/arm64/kvm/vgic/vgic-v5.c
> @@ -8,6 +8,8 @@
>  
>  #include "vgic.h"
>  
> +static struct vgic_v5_ppi_caps *ppi_caps;
> +
>  /*
>   * Probe for a vGICv5 compatible interrupt controller, returning 0 on success.
>   * Currently only supports GICv3-based VMs on a GICv5 host, and hence only
> @@ -53,3 +55,34 @@ int vgic_v5_probe(const struct gic_kvm_info *info)
>  
>  	return 0;
>  }
> +
> +/*
> + * Not all PPIs are guaranteed to be implemented for GICv5. Deterermine which
> + * ones are, and generate a mask.
> + */
> +void vgic_v5_get_implemented_ppis(void)
> +{
> +	if (!cpus_have_final_cap(ARM64_HAS_GICV5_CPUIF))
> +		return;
> +
> +	/* Never freed again */
> +	ppi_caps = kzalloc(sizeof(*ppi_caps), GFP_KERNEL);
> +	if (!ppi_caps)
> +		return;

Maybe we can spare the call by statically allocating the PPI
structure? Just the code calling kzalloc() costs us more than the 128
bits required by the structure.

Thanks,

	M.

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