Re: [PATCH v2 1/2] KVM: x86: Check hypercall's exit to userspace generically

[Yuan Yao]

On Tue, Aug 13, 2024 at 01:12:55PM +0800, Binbin Wu wrote:
> Check whether a KVM hypercall needs to exit to userspace or not based on
> hypercall_exit_enabled field of struct kvm_arch.
>
> Userspace can request a hypercall to exit to userspace for handling by
> enable KVM_CAP_EXIT_HYPERCALL and the enabled hypercall will be set in
> hypercall_exit_enabled.  Make the check code generic based on it.
>
> Signed-off-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 4 ++--
>  arch/x86/kvm/x86.h | 7 +++++++
>  2 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index af6c8cf6a37a..6e16c9751af7 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10226,8 +10226,8 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>  	cpl = kvm_x86_call(get_cpl)(vcpu);
>
>  	ret = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl);
> -	if (nr == KVM_HC_MAP_GPA_RANGE && !ret)
> -		/* MAP_GPA tosses the request to the user space. */
> +	if (!ret && is_kvm_hc_exit_enabled(vcpu->kvm, nr))
> +		/* The hypercall is requested to exit to userspace. */
>  		return 0;
>
>  	if (!op_64_bit)
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 50596f6f8320..0cbec76b42e6 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -547,4 +547,11 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
>  			 unsigned int port, void *data,  unsigned int count,
>  			 int in);
>
> +static inline bool is_kvm_hc_exit_enabled(struct kvm *kvm, unsigned long hc_nr)
> +{
> +	if(WARN_ON_ONCE(hc_nr >= sizeof(kvm->arch.hypercall_exit_enabled) * 8))

How about:

if (!(BIT(hc_nr) & KVM_EXIT_HYPERCALL_VALID_MASK))

KVM_EXIT_HYPERCALL_VALID_MASK is used to guard kvm->arch.hypercall_exit_enabled
on KVM_CAP_EXIT_HYPERCALL, "hc_nr > maximum supported hc" AND "hc_nr <=
bit_count(kvm->arch.hypercall_exit_enabled)" should be treated as invalid yet to
me.

> +		return false;
> +
> +	return kvm->arch.hypercall_exit_enabled & (1 << hc_nr);

BIT(xx) instead of "1 << hc_nr" for better readability.

> +}
>  #endif
> --
> 2.43.2
>
>