Re: [PATCH kvmtool 5/5] arm64: Query counter-offset support on the VM fd

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 31/08/2026 20:24, Fuad Tabba wrote:
> kvm__arch_set_counter_offset() probes KVM_CAP_COUNTER_OFFSET with
> kvm__supports_extension(), which issues KVM_CHECK_EXTENSION on the
> global /dev/kvm fd. That reports the host's raw capabilities, unaware of
> any per-VM restrictions.
> 
> pKVM does not offer the counter offset to a protected VM, and reflects
> that on the VM fd alone. kvmtool's own check passes, and the
> KVM_ARM_SET_COUNTER_OFFSET that follows is refused by
> commit b12b3b04f6ba ("KVM: arm64: Check whether a VM IOCTL is allowed
> in pKVM"):
> 
>    KVM_ARM_SET_COUNTER_OFFSET: Invalid argument
> 
> Query it on the VM fd via kvm__supports_vm_extension(), so
> --counter-offset on a protected VM fails with kvmtool's own message:
> 
>    Fatal: No support for global counter offset
> 
> Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>


> ---
>   arm64/kvm.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arm64/kvm.c b/arm64/kvm.c
> index 36b3284..0bb3535 100644
> --- a/arm64/kvm.c
> +++ b/arm64/kvm.c
> @@ -155,7 +155,7 @@ static void kvm__arch_set_counter_offset(struct kvm *kvm)
>   	if (!kvm->cfg.arch.counter_offset)
>   		return;
>   
> -	if (!kvm__supports_extension(kvm, KVM_CAP_COUNTER_OFFSET))
> +	if (!kvm__supports_vm_extension(kvm, KVM_CAP_COUNTER_OFFSET))
>   		die("No support for global counter offset");
>   
>   	if (ioctl(kvm->vm_fd, KVM_ARM_SET_COUNTER_OFFSET, &offset))