KVM: arm64: vgic-v3: Allow access to GICD_IIDR prior to initialization

KVM allows userspace to write GICD_IIDR for backwards-compatibility with
older kernels, where new implementation revisions have new features.
Unfortunately this is allowed to happen at runtime, and ripping features
out from underneath a running guest is a terrible idea.

While we can't do anything about the ABI, prepare for more ID-like
registers by allowing access to GICD_IIDR prior to VGIC initialization.
Hoist initializaiton of the default value to kvm_vgic_create() and
discard the incorrect comment that assumed userspace could access the
register before initialization (until now).

Subsequent changes will allow the VMM to further provision the GIC
feature set, e.g. the presence of nASSGIcap.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250724062805.2658919-4-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

1 files changed, 1 insertions, 8 deletions

diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
index 0d8bc2ac627a..31462ba093c9 100644
--- a/arch/arm64/kvm/vgic/vgic-init.c
+++ b/arch/arm64/kvm/vgic/vgic-init.c
@@ -157,6 +157,7 @@ int kvm_vgic_create(struct kvm *kvm, u32 type)
 
 	kvm->arch.vgic.in_kernel = true;
 	kvm->arch.vgic.vgic_model = type;
+	kvm->arch.vgic.implementation_rev = KVM_VGIC_IMP_REV_LATEST;
 
 	kvm->arch.vgic.vgic_dist_base = VGIC_ADDR_UNDEF;
 
@@ -408,15 +409,7 @@ int vgic_init(struct kvm *kvm)
 		goto out;
 
 	vgic_debug_init(kvm);
-
-	/*
-	 * If userspace didn't set the GIC implementation revision,
-	 * default to the latest and greatest. You know want it.
-	 */
-	if (!dist->implementation_rev)
-		dist->implementation_rev = KVM_VGIC_IMP_REV_LATEST;
 	dist->initialized = true;
-
 out:
 	return ret;
 }