Re: [PATCH v13 32/48] arm64: Don't expose stolen time for realm guests

Suzuki K Poulose
2026-03-30T10:52:53Z

On 18/03/2026 15:53, Steven Price wrote:
> It doesn't make much sense as a realm guest wouldn't want to trust the
> host. It will also need some extra work to ensure that KVM will only
> attempt to write into a shared memory region. So for now just disable
> it.
>
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
> Changes since v7:
> * Update the documentation to add a note about stolen time being
> unavailable in a realm.
> ---
> Documentation/virt/kvm/api.rst | 3 +++
> arch/arm64/kvm/arm.c | 5 ++++-
> 2 files changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index bc180c853faf..70911fe6d435 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -9240,6 +9240,9 @@ is supported, than the other should as well and vice versa. For arm64
> see Documentation/virt/kvm/devices/vcpu.rst "KVM_ARM_VCPU_PVTIME_CTRL".
> For x86 see Documentation/virt/kvm/x86/msr.rst "MSR_KVM_STEAL_TIME".
>
> +Note that steal time accounting is not available when a guest is running
> +within a Arm CCA realm (machine type KVM_VM_TYPE_ARM_REALM).
> +
> 8.25 KVM_CAP_S390_DIAG318
> -------------------------
>
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 61182eb0cf70..7d92ddb06460 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -469,7 +469,10 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> r = system_supports_mte();
> break;
> case KVM_CAP_STEAL_TIME:
> - r = kvm_arm_pvtime_supported();
> + if (kvm_is_realm(kvm))
> + r = 0;
> + else
> + r = kvm_arm_pvtime_supported();

Could this be handled in kvm_realm_ext_allowed() ?

Suzuki

> break;
> case KVM_CAP_ARM_EL1_32BIT:
> r = cpus_have_final_cap(ARM64_HAS_32BIT_EL1);