Re: [PATCH v6 3/4] KVM: arm64: Report all the KVM/arm64-specific hypercalls

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

On Wed, Feb 05, 2025 at 01:22:21PM +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..5cef2590ffdf 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_PTP,
> +				 ARM_SMCCC_KVM_FUNC_FEATURES);
> +		/* Function numbers 2-63 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, vendor_hyp_bmap is very much load bearing. We have a
documented UAPI that allows userspace to control the hypercalls exposed
to the guest.

The idea being a user wants kernel rollback safety and doesn't expose
hypercalls that could potentially be revoked.

https://docs.kernel.org/virt/kvm/arm/fw-pseudo-registers.html#bitmap-feature-firmware-registers

-- 
Thanks,
Oliver