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

[Bradley Morgan]

On July 17, 2026 2:03:16 PM GMT+01:00, Fuad Tabba <fuad.tabba@xxxxxxxxx>
wrote:
>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@xxxxxxxxx>

Do you think CC stable would be good? I do but sometimes I'm
embarrassingly wrong

>---
> 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;


Nit: KVM_CAP_ARM_MTE right above already returns false, fold the two
new cases into that and drop the duplicate return. And please put a small
comment saying eager splitting is unimplemented under pKVM (WARN
stub), so nobody reads the list later and "fixes" it back, here's a
example, feel free to bikeshed


	case KVM_CAP_ARM_MTE:
	/* eager splitting is unimplemented in pKVM, please don't refix this */
	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);
	}




Killing the reporting too is right imo. Qemu reads a 0 block size
bitmap and quietly falls back, an explicit enable eating EINVAL is the
worse failure mode. Good.

if u choose not to, it's ok, please add 

Reviewed-by: Bradley Morgan
<include@xxxxxxxxx>


> 	default:
> 		return !kvm || !kvm_vm_is_protected(kvm);
> 	}
>

Thanks!