[PATCH v5 6/7] KVM: arm64: Don't advertise eager page splitting under pKVM

[Fuad Tabba <fuad.tabba@linux.dev>]

Under pKVM the stage-2 walker resolves to pkvm_pgtable_stage2_split(), a
WARN_ON_ONCE(1) stub, yet KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE is still
enabled and reported for non-protected guests: the capability check keys
on the per-VM protected state while the walker dispatch keys on the
host-global mode. Enabling the cap and then dirty-logging the guest
reaches the stub, splatting a userspace-reachable WARN.

Reject the capability, and stop reporting a chunk size and the
supported block sizes, for every VM once pKVM is enabled, keyed on the
host-global mode like the split dispatch. Gating only protected VMs
would leave the non-protected guests that reach the stub still able to
enable it. Userspace decides whether eager splitting is available from
the block-size bitmap (QEMU falls back to no eager splitting when it
reads 0), so leaving it advertised steers an explicit request into the
enable failure instead of the fallback.

Fixes: e912efed485a ("KVM: arm64: Introduce the EL1 pKVM MMU")
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 arch/arm64/include/asm/kvm_pkvm.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_pkvm.h b/arch/arm64/include/asm/kvm_pkvm.h
index 57afb07d6b13..beea00e693a0 100644
--- a/arch/arm64/include/asm/kvm_pkvm.h
+++ b/arch/arm64/include/asm/kvm_pkvm.h
@@ -45,6 +45,9 @@ static inline bool kvm_pkvm_ext_allowed(struct kvm *kvm, long ext)
 		return true;
 	case KVM_CAP_ARM_MTE:
 		return false;
+	case KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE:
+	case KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES:
+		return false;
 	default:
 		return !kvm || !kvm_vm_is_protected(kvm);
 	}
-- 
2.39.5