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

[Ben Horgan <ben.horgan@arm.com>]

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.
> 
> On the other hand, if KVM has the vGICv4 enabled via the cmdline, the
> VM absorbs this configuration by default to maintain the backward
> compatibility. Userspace can get the attribute's value to check if the VM
> has vGICv4 support if it sees KVM_DEV_ARM_VGIC_CONFIG_GICV4_ENABLE as the
> value. As required, it can disable vGICv4 by setting
> KVM_DEV_ARM_VGIC_CONFIG_GICV4_DISABLE as the value.
> 
...
> 
> 

Thanks,

Ben