linux-kernel - Re: [PATCH 0/3] KVM: arm64: Allow vGICv4 configuration per VM

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Message-ID: <CAJHc60w1rYc9guoideuKpKaukuCyvxu3S7Fidoy3Lh94+_xDiw@mail.gmail.com>
Date: Thu, 15 May 2025 08:55:47 -0700
From: Raghavendra Rao Ananta <rananta@...gle.com>
To: Ben Horgan <ben.horgan@....com>
Cc: Oliver Upton <oliver.upton@...ux.dev>, Marc Zyngier <maz@...nel.org>, 
	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 0/3] KVM: arm64: Allow vGICv4 configuration per VM

On Thu, May 15, 2025 at 3:30 AM Ben Horgan <ben.horgan@....com> wrote:
>
> Hi,
>
> On 5/14/25 20:21, Raghavendra Rao Ananta wrote:
> > Hello,
> >
> > When kvm-arm.vgic_v4_enable=1, KVM adds support for direct interrupt
> > injection by default to all the VMs in the system, aka GICv4. A
> > shortcoming of the GIC architecture is that there's an absolute limit on
> > the number of vPEs that can be tracked by the ITS. It is possible that
> > an operator is running a mix of VMs on a system, only wanting to provide
> > a specific class of VMs with hardware interrupt injection support.
> >
> > To support this, introduce a GIC attribute, KVM_DEV_ARM_VGIC_CONFIG_GICV4,
> > for the userspace to enable or disable vGICv4 for a given VM.
> >
> > The attribute allows the configuration only when vGICv4 is enabled in KVM,
> > else it acts a read-only attribute returning
> > KVM_DEV_ARM_VGIC_CONFIG_GICV4_UNAVAILABLE as the value.
> What's the reason for the cmdline enable continuing to be absolute in
> the disable case? I wonder if this is unnecessarily restrictive.
>
> Couldn't KVM_DEV_ARM_VGIC_CONFIG_GICV4_UNAVAILABLE be reserved for
> hardware that doesn't support vgic_v4 and if kvm-arm.vgic_v4_enable=0,
> or omitted, on supporting hardware then default to
> KVM_DEV_ARM_VGIC_CONFIG_GICV4_DISABLE but allow it to be overridden? I
> don't think this changes the behaviour when your new attribute is not used.

KVM_DEV_ARM_VGIC_CONFIG_GICV4_UNAVAILABLE is reserved for the exact
situation that you mentioned (no GICv4 h/w support  or if cmdline is
disabled/omitted).
Regarding defaulting to KVM_DEV_ARM_VGIC_CONFIG_GICV4_DISABLE,
wouldn't it change the existing expectations, i.e., vGICv4 is enabled
if available and set by cmdline?

Thank you.
Raghavendra