Re: [PATCH v5 3/4] KVM: arm64: Report all the KVM/arm64-specific hypercalls

[Oliver Upton <oliver.upton@linux.dev>]

Hi Shameer,

On Fri, Jan 24, 2025 at 03:17:31PM +0000, Shameer Kolothum wrote:
> Currently ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID returns the
> bitmap corresponding to KVM_REG_ARM_VENDOR_HYP_BMAP and it only
> returns _KVM_FEATURES_FUNC_ID and _KVM_PTP_FUNC_ID. Change that
> to return all the KVM/arm64-specific hypercalls exposed by
> KVM/arm64 to guest operating systems.
> 
> Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
> ---
>  arch/arm64/kvm/hypercalls.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> index 27ce4cb44904..6132cb542200 100644
> --- a/arch/arm64/kvm/hypercalls.c
> +++ b/arch/arm64/kvm/hypercalls.c
> @@ -359,7 +359,11 @@ int kvm_smccc_call_handler(struct kvm_vcpu *vcpu)
>  		val[3] = ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_3;
>  		break;
>  	case ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID:
> -		val[0] = smccc_feat->vendor_hyp_bmap;
> +		val[0] = GENMASK(ARM_SMCCC_KVM_FUNC_MMIO_GUARD,
> +				 ARM_SMCCC_KVM_FUNC_FEATURES);
> +		/* Function numbers 8-63 are reserved for pKVM for now */
> +		val[2] = GENMASK((ARM_SMCCC_KVM_FUNC_DISCOVER_IMPL_CPUS - 64),
> +				 (ARM_SMCCC_KVM_FUNC_DISCOVER_IMPL_VER - 64));
>  		break;

This isn't right. The pKVM carveout exists for some KVM-internal
bookkeeping to (hopefully) avoid breaking guest ABI between what's in
the downstream android kernel and what eventually gets accepted
upstream.

The purpose of this hypercall is for the guest to discover what
interfaces are actually implemented by the hypervisor, and we definitely
do not implement these upstream at the moment.

-- 
Thanks,
Oliver