Re: [PATCH v2 09/11] KVM: arm64: Type-check hypercall arguments at the caller

On Mon, 3 Aug 2026 at 18:59, Marc Zyngier <[email protected]> wrote:
...
> > +/*
> > + * Generate a typed stub for each declared hypercall. kvm_call_hyp_nvhe()
> > + * resolves to the stub, so a call with the wrong argument count or types
> > + * fails to compile instead of being silently truncated to an SMCCC function
> > + * number and a pile of registers. The stub inlines to the same SMCCC call
> > + * the untyped macro used to make.
> > + */
> > +#define DECLARE_KVM_HOST_HCALL(ret, name, ...)                               \
> > +     static __always_inline                                          \
> > +     ret nvhe_hvc_##name(__KVM_HCALL_MAP(__KVM_HCALL_DECL, __VA_ARGS__)) \
> > +     {                                                               \
> > +             return (ret)__kvm_call_hyp_nvhe(name,                   \
> > +                     __KVM_HCALL_MAP(__KVM_HCALL_ARGS, __VA_ARGS__));\
> > +     }
> > +
> > +#define DECLARE_KVM_HOST_HCALL_VOID(name, ...)                               \
> > +     static __always_inline                                          \
> > +     void nvhe_hvc_##name(__KVM_HCALL_MAP(__KVM_HCALL_DECL, __VA_ARGS__)) \
> > +     {                                                               \
> > +             __kvm_call_hyp_nvhe(name,                               \
> > +                     __KVM_HCALL_MAP(__KVM_HCALL_ARGS, __VA_ARGS__));\
> > +     }
>
> I'll bite! ;-) void is a type, not the absence of a type. And
> returning void is perfectly valid.
>
> So with this in mind, I got rid of the *_VOID() helpers altogether,
> see below.

This looks better. I've built and boot tested it, and the generated
code is identical.

I noticed is that gcc and clang complain about "return (void)expr"
under -Wpedantic, since ISO C only allows it from C23. We never build
with that, right? So it doesn't bite ;)

Cheers,
/fuad



>
> Thanks,
>
>         M.
>
> diff --git a/arch/arm64/include/asm/kvm_hcall.h b/arch/arm64/include/asm/kvm_hcall.h
> index 3958396a83345..0b1b10d4212da 100644
> --- a/arch/arm64/include/asm/kvm_hcall.h
> +++ b/arch/arm64/include/asm/kvm_hcall.h
> @@ -88,26 +88,12 @@ struct vgic_v5_cpu_if;
>                         __KVM_HCALL_MAP(__KVM_HCALL_ARGS, __VA_ARGS__));\
>         }
>
> -#define DECLARE_KVM_HOST_HCALL_VOID(name, ...)                         \
> -       static __always_inline                                          \
> -       void nvhe_hvc_##name(__KVM_HCALL_MAP(__KVM_HCALL_DECL, __VA_ARGS__)) \
> -       {                                                               \
> -               __kvm_call_hyp_nvhe(name,                               \
> -                       __KVM_HCALL_MAP(__KVM_HCALL_ARGS, __VA_ARGS__));\
> -       }
> -
>  #define DECLARE_KVM_HOST_HCALL0(ret, name)                             \
>         static __always_inline ret nvhe_hvc_##name(void)                \
>         {                                                               \
>                 return (ret)__kvm_call_hyp_nvhe(name);                  \
>         }
>
> -#define DECLARE_KVM_HOST_HCALL0_VOID(name)                             \
> -       static __always_inline void nvhe_hvc_##name(void)               \
> -       {                                                               \
> -               __kvm_call_hyp_nvhe(name);                              \
> -       }
> -
>  #define kvm_call_hyp_nvhe(f, ...)      nvhe_hvc_##f(__VA_ARGS__)
>
>  /*
> @@ -148,12 +134,8 @@ struct vgic_v5_cpu_if;
>   */
>  #define DECLARE_KVM_HOST_HCALL(ret, name, ...)                         \
>         typedef ret kvm_host_hcall_sig_##name(__KVM_HCALL_MAP(__KVM_HCALL_DECL, __VA_ARGS__));
> -#define DECLARE_KVM_HOST_HCALL_VOID(name, ...)                         \
> -       typedef void kvm_host_hcall_sig_##name(__KVM_HCALL_MAP(__KVM_HCALL_DECL, __VA_ARGS__));
>  #define DECLARE_KVM_HOST_HCALL0(ret, name)                             \
>         typedef ret kvm_host_hcall_sig_##name(void);
> -#define DECLARE_KVM_HOST_HCALL0_VOID(name)                             \
> -       typedef void kvm_host_hcall_sig_##name(void);
>  #endif /* __KVM_NVHE_HYPERVISOR__ */
>
>  /* Hypercalls that are unavailable once pKVM has finalised. */
> @@ -164,52 +146,52 @@ DECLARE_KVM_HOST_HCALL(unsigned long, __pkvm_create_private_mapping,
>         phys_addr_t, phys, size_t, size, u64, prot)
>  DECLARE_KVM_HOST_HCALL(int, __pkvm_cpu_set_vector,
>         enum arm64_hyp_spectre_vector, slot)
> -DECLARE_KVM_HOST_HCALL0_VOID(__kvm_enable_ssbs)
> -DECLARE_KVM_HOST_HCALL0_VOID(__vgic_v3_init_lrs)
> +DECLARE_KVM_HOST_HCALL0(void, __kvm_enable_ssbs)
> +DECLARE_KVM_HOST_HCALL0(void, __vgic_v3_init_lrs)
>  DECLARE_KVM_HOST_HCALL0(u64, __vgic_v3_get_gic_config)
>
>  DECLARE_KVM_HOST_HCALL0(int, __pkvm_prot_finalize)
>
>  /* Hypercalls that are always available and common to [nh]VHE/pKVM. */
> -DECLARE_KVM_HOST_HCALL_VOID(__kvm_adjust_pc,
> +DECLARE_KVM_HOST_HCALL(void, __kvm_adjust_pc,
>         struct kvm_vcpu __kern *, vcpu)
>  DECLARE_KVM_HOST_HCALL(int, __kvm_vcpu_run,
>         struct kvm_vcpu __kern *, vcpu)
> -DECLARE_KVM_HOST_HCALL0_VOID(__kvm_flush_vm_context)
> -DECLARE_KVM_HOST_HCALL_VOID(__kvm_tlb_flush_vmid_ipa,
> +DECLARE_KVM_HOST_HCALL0(void, __kvm_flush_vm_context)
> +DECLARE_KVM_HOST_HCALL(void, __kvm_tlb_flush_vmid_ipa,
>         struct kvm_s2_mmu __kern *, mmu, phys_addr_t, ipa, int, level)
> -DECLARE_KVM_HOST_HCALL_VOID(__kvm_tlb_flush_vmid_ipa_nsh,
> +DECLARE_KVM_HOST_HCALL(void, __kvm_tlb_flush_vmid_ipa_nsh,
>         struct kvm_s2_mmu __kern *, mmu, phys_addr_t, ipa, int, level)
> -DECLARE_KVM_HOST_HCALL_VOID(__kvm_tlb_flush_vmid,
> +DECLARE_KVM_HOST_HCALL(void, __kvm_tlb_flush_vmid,
>         struct kvm_s2_mmu __kern *, mmu)
> -DECLARE_KVM_HOST_HCALL_VOID(__kvm_tlb_flush_vmid_range,
> +DECLARE_KVM_HOST_HCALL(void, __kvm_tlb_flush_vmid_range,
>         struct kvm_s2_mmu __kern *, mmu, phys_addr_t, start, unsigned long, pages)
> -DECLARE_KVM_HOST_HCALL_VOID(__kvm_flush_cpu_context,
> +DECLARE_KVM_HOST_HCALL(void, __kvm_flush_cpu_context,
>         struct kvm_s2_mmu __kern *, mmu)
> -DECLARE_KVM_HOST_HCALL_VOID(__kvm_timer_set_cntvoff,
> +DECLARE_KVM_HOST_HCALL(void, __kvm_timer_set_cntvoff,
>         u64, cntvoff)
>  DECLARE_KVM_HOST_HCALL(int, __tracing_load,
>         void __kern *, desc_hva, size_t, desc_size)
> -DECLARE_KVM_HOST_HCALL0_VOID(__tracing_unload)
> +DECLARE_KVM_HOST_HCALL0(void, __tracing_unload)
>  DECLARE_KVM_HOST_HCALL(int, __tracing_enable,
>         bool, enable)
>  DECLARE_KVM_HOST_HCALL(int, __tracing_swap_reader,
>         unsigned int, cpu)
> -DECLARE_KVM_HOST_HCALL_VOID(__tracing_update_clock,
> +DECLARE_KVM_HOST_HCALL(void, __tracing_update_clock,
>         u32, mult, u32, shift, u64, epoch_ns, u64, epoch_cyc)
>  DECLARE_KVM_HOST_HCALL(int, __tracing_reset,
>         unsigned int, cpu)
>  DECLARE_KVM_HOST_HCALL(int, __tracing_enable_event,
>         unsigned short, id, bool, enable)
> -DECLARE_KVM_HOST_HCALL_VOID(__tracing_write_event,
> +DECLARE_KVM_HOST_HCALL(void, __tracing_write_event,
>         u64, id)
> -DECLARE_KVM_HOST_HCALL_VOID(__vgic_v3_save_aprs,
> +DECLARE_KVM_HOST_HCALL(void, __vgic_v3_save_aprs,
>         struct vgic_v3_cpu_if __kern *, cpu_if)
> -DECLARE_KVM_HOST_HCALL_VOID(__vgic_v3_restore_vmcr_aprs,
> +DECLARE_KVM_HOST_HCALL(void, __vgic_v3_restore_vmcr_aprs,
>         struct vgic_v3_cpu_if __kern *, cpu_if)
> -DECLARE_KVM_HOST_HCALL_VOID(__vgic_v5_save_apr,
> +DECLARE_KVM_HOST_HCALL(void, __vgic_v5_save_apr,
>         struct vgic_v5_cpu_if __kern *, cpu_if)
> -DECLARE_KVM_HOST_HCALL_VOID(__vgic_v5_restore_vmcr_apr,
> +DECLARE_KVM_HOST_HCALL(void, __vgic_v5_restore_vmcr_apr,
>         struct vgic_v5_cpu_if __kern *, cpu_if)
>
>  /* Hypercalls that are available only when pKVM has finalised. */
> @@ -232,7 +214,7 @@ DECLARE_KVM_HOST_HCALL(int, __pkvm_host_test_clear_young_guest,
>  DECLARE_KVM_HOST_HCALL(int, __pkvm_host_mkyoung_guest,
>         u64, gfn)
>  DECLARE_KVM_HOST_HCALL0(int, __pkvm_reserve_vm)
> -DECLARE_KVM_HOST_HCALL_VOID(__pkvm_unreserve_vm,
> +DECLARE_KVM_HOST_HCALL(void, __pkvm_unreserve_vm,
>         pkvm_handle_t, handle)
>  DECLARE_KVM_HOST_HCALL(int, __pkvm_init_vm,
>         struct kvm __kern *, host_kvm, void __kern *, vm_hva,
> @@ -249,10 +231,10 @@ DECLARE_KVM_HOST_HCALL(int, __pkvm_start_teardown_vm,
>         pkvm_handle_t, handle)
>  DECLARE_KVM_HOST_HCALL(int, __pkvm_finalize_teardown_vm,
>         pkvm_handle_t, handle)
> -DECLARE_KVM_HOST_HCALL_VOID(__pkvm_vcpu_load,
> +DECLARE_KVM_HOST_HCALL(void, __pkvm_vcpu_load,
>         pkvm_handle_t, handle, unsigned int, vcpu_idx, u64, hcr_el2)
> -DECLARE_KVM_HOST_HCALL0_VOID(__pkvm_vcpu_put)
> -DECLARE_KVM_HOST_HCALL_VOID(__pkvm_tlb_flush_vmid,
> +DECLARE_KVM_HOST_HCALL0(void, __pkvm_vcpu_put)
> +DECLARE_KVM_HOST_HCALL(void, __pkvm_tlb_flush_vmid,
>         pkvm_handle_t, handle)
>
>  #endif /* __ARM64_KVM_HCALL_H__ */
>
> --
> Without deviation from the norm, progress is not possible.