linux-kernel - Re: [PATCH v3 3/4] KVM: arm64: Introduce attribute to control GICD

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Message-ID: <aFkTDmj9u1ERnvHO@linux.dev>
Date: Mon, 23 Jun 2025 01:40:46 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Marc Zyngier <maz@...nel.org>
Cc: Raghavendra Rao Ananta <rananta@...gle.com>,
	Mingwei Zhang <mizhang@...gle.com>,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 3/4] KVM: arm64: Introduce attribute to control
 GICD_TYPER2.nASSGIcap

On Sat, Jun 21, 2025 at 09:50:48AM +0100, Marc Zyngier wrote:
> On Fri, 13 Jun 2025 16:52:37 +0100, Raghavendra Rao Ananta <rananta@...gle.com> wrote:
> > @@ -683,8 +714,14 @@ static int vgic_v3_has_attr(struct kvm_device *dev,
> >  			return 0;
> >  		case KVM_DEV_ARM_VGIC_SAVE_PENDING_TABLES:
> >  			return 0;
> > +		default:
> > +			return -ENXIO;
> >  		}
> > +	case KVM_DEV_ARM_VGIC_GRP_FEATURES:
> > +		return attr->attr != KVM_DEV_ARM_VGIC_FEATURE_nASSGIcap ?
> > +		       -ENXIO : 0;
> 
> Do we really want to advertise KVM_DEV_ARM_VGIC_FEATURE_nASSGIcap even
> when we don't have GICv4.1? This seems rather odd. My take on this API
> is that this should report whether the feature is configurable, making
> it backward compatible with older versions of KVM.

So this was because of me, as I wanted nASSGIcap to behave exactly like
the ID registers. I do think exposing the capability unconditionally is
useful, as otherwise there's no way to definitively say whether or not
the underlying platform supports GICv4.1.

KVM_HAS_DEVICE_ATTR can't be used alone for probing since old kernels
use GICv4.1 but don't expose the attribute.

Does that make sense?

Thanks,
Oliver