Re: [PATCH 0/5] KVM: arm64: GICv5 legacy (GCIE_LEGACY) NV enablement and cleanup

On Thu, Aug 28, 2025 at 10:59:41AM +0000, Sascha Bischoff wrote:

Hi all,

This series enables nested virtualization for GICv3-based VMs on GICv5
hosts (w/ FEAT_GCIE_LEGACY) in KVM/arm64. In addition, it adds a CPU
capability to track support for FEAT_GCIE_LEGACY across all CPUs.

The series fixes ICC_SRE_EL2 access handling for GICv5 hosts (to match
the updated bet1+ specification [1]), and extends nested
virtualization support to vGICv3 guests running on compatible GICv5
systems. With these changes, it becomes possible to run with
kvm-arm.mode=nested, and these changes have been tested with three
levels of nesting on simulated hardware (Arm FVP).

Previously, the presence of FEAT_GCIE_LEGACY was tracked in the GICv5
driver via gic_kvm_info, and the probing logic could incorrectly
enable legacy support if the boot CPU exposed the feature while others
did not. This created the risk of mismatched configurations,
particularly when late-onlining CPUs without FEAT_GCIE_LEGACY.

To address this, the series introduces a proper ARM64_HAS_GICV5_LEGACY
CPU capability, and moves KVM to use cpus_have_final_cap() to ensure
consistent system-wide enablement. With this, late-onlined but
mismatched CPUs are cleanly rejected at bring-up.

Patch summary

KVM: arm64: allow ICC_SRE_EL2 accesses on a GICv5 host
    Update handling to reflect the corrected GICv5 specification.

KVM: arm64: Enable nested for GICv5 host with FEAT_GCIE_LEGACY
    Allow nested virtualization for vGICv3 guests on GICv5 hosts with
    legacy support.

arm64: cpucaps: Add GICv5 Legacy vCPU interface (GCIE_LEGACY) capability
    Introduce a new CPU capability that prevents mismatched
    configurations.

KVM: arm64: Use ARM64_HAS_GICV5_LEGACY for GICv5 probing
    Ensure probing is consistent across all CPUs by using cpucaps.

irqchip/gic-v5: Drop has_gcie_v3_compat from gic_kvm_info
    Remove obsolete compatibility flag, as FEAT_GCIE_LEGACY is now a
    CPU feature.

Comments and reviews are very welcome.

Thanks,
Sascha

[1] https://developer.arm.com/documentation/aes0070/latest/

Sascha Bischoff (5):
  KVM: arm64: Allow ICC_SRE_EL2 accesses on a GICv5 host
  KVM: arm64: Enable nested for GICv5 host with FEAT_GCIE_LEGACY
  arm64: cpucaps: Add GICv5 Legacy vCPU interface (GCIE_LEGACY)
    capability
  KVM: arm64: Use ARM64_HAS_GICV5_LEGACY for GICv5 probing
  irqchip/gic-v5: Drop has_gcie_v3_compat from gic_kvm_info

For the series:

Reviewed-by: Oliver Upton <redacted>

Thanks,
Oliver