Re: [PATCH v1 2/3] x86/kvm/hyper-v: enable hypercalls regardless of hypercall page

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Jon Doron <arilou@gmail.com> writes:

> Microsoft's kdvm.dll dbgtransport module does not respect the hypercall
> page and simply identifies the CPU being used (AMD/Intel) and according
> to it simply makes hypercalls with the relevant instruction
> (vmmcall/vmcall respectively).
>
> The relevant function in kdvm is KdHvConnectHypervisor which first checks
> if the hypercall page has been enabled via HV_X64_MSR_HYPERCALL_ENABLE,
> and in case it was not it simply sets the HV_X64_MSR_GUEST_OS_ID to
> 0x1000101010001 which means:
> build_number = 0x0001
> service_version = 0x01
> minor_version = 0x01
> major_version = 0x01
> os_id = 0x00 (Undefined)
> vendor_id = 1 (Microsoft)
> os_type = 0 (A value of 0 indicates a proprietary, closed source OS)
>
> and starts issuing the hypercall without setting the hypercall page.
>
> To resolve this issue simply enable hypercalls if the guest_os_id is
> not 0.
>
> Signed-off-by: Jon Doron <arilou@gmail.com>
> ---
>  arch/x86/kvm/hyperv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 13176ec23496..7ec962d433af 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1615,7 +1615,7 @@ static u64 kvm_hv_send_ipi(struct kvm_vcpu *current_vcpu, u64 ingpa, u64 outgpa,
>  
>  bool kvm_hv_hypercall_enabled(struct kvm *kvm)
>  {
> -	return READ_ONCE(kvm->arch.hyperv.hv_hypercall) & HV_X64_MSR_HYPERCALL_ENABLE;
> +	return READ_ONCE(kvm->arch.hyperv.hv_guest_os_id) != 0;
>  }
>  
>  static void kvm_hv_hypercall_set_result(struct kvm_vcpu *vcpu, u64 result)

I would've enabled it in both cases,

return (READ_ONCE(kvm->arch.hyperv.hv_hypercall) &
 HV_X64_MSR_HYPERCALL_ENABLE) || (READ_ONCE(kvm->arch.hyperv.hv_guest_os_id) != 0);

to be safe. We can also check what genuine Hyper-V does but I bet it has
hypercalls always enabled. Also, the function can be made inline,
there's a single caller.

-- 
Vitaly