Re: [PATCH] KVM: Hyper-V: do not do hypercall userspace exits if SynIC is disabled

[Roman Kagan <rkagan@virtuozzo.com>]

On Tue, Mar 29, 2016 at 11:34:56AM +0200, Paolo Bonzini wrote:
> If SynIC is disabled, there is nothing that userspace can do to
> handle these exits; on the other hand, userspace probably will
> not know about KVM_EXIT_HYPERV_HCALL and complain about it or
> even exit.  Just prevent anything bad from happening by handling
> the hypercall in KVM and returning an "invalid hypercall" code.

I wonder if this has been encountered in real life or just found by code
inspection?

> Fixes: 83326e43f27e9a8a501427a0060f8af519a39bb2
> Cc: Andrey Smetanin <asmetanin@virtuozzo.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/hyperv.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index b960d9ea171f..9e2becd20a59 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1237,14 +1237,17 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>  		break;
>  	case HVCALL_POST_MESSAGE:
>  	case HVCALL_SIGNAL_EVENT:
> -		vcpu->run->exit_reason = KVM_EXIT_HYPERV;
> -		vcpu->run->hyperv.type = KVM_EXIT_HYPERV_HCALL;
> -		vcpu->run->hyperv.u.hcall.input = param;
> -		vcpu->run->hyperv.u.hcall.params[0] = ingpa;
> -		vcpu->run->hyperv.u.hcall.params[1] = outgpa;
> -		vcpu->arch.complete_userspace_io =
> +		if (vcpu_to_synic(vcpu)->active) {
> +			vcpu->run->exit_reason = KVM_EXIT_HYPERV;
> +			vcpu->run->hyperv.type = KVM_EXIT_HYPERV_HCALL;
> +			vcpu->run->hyperv.u.hcall.input = param;
> +			vcpu->run->hyperv.u.hcall.params[0] = ingpa;
> +			vcpu->run->hyperv.u.hcall.params[1] = outgpa;
> +			vcpu->arch.complete_userspace_io =
>  				kvm_hv_hypercall_complete_userspace;
> -		return 0;
> +			return 0;
> +		}
> +		/* fall through */

I'd rather put it the other way around, with explicit error path and a
comment:


		/* don't bother userspace if it has no way to handle it */
		if (!vcpu_to_synic(vcpu)->active) {
			res = HV_STATUS_INVALID_HYPERCALL_CODE;
			break;
		}

but that's just nitpicking, so

Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>

[ Andrey has left the company so I'm not sure he even receives the
messages sent to this address ]

Thanks,
Roman.