KVM: selftests: Load RAX with -EFAULT before Hyper-V hypercall

Load RAX with -EFAULT prior to making a Hyper-V hypercall so that tests
can't get false negatives due to the compiler coincidentally loading the
"right" value into RAX, i.e. to ensure that _KVM_ and not the compiler
is correctly clearing RAX on a successful hypercall.

Note, initializing *hv_status (in C code) to -EFAULT is not sufficient
to avoid false negatives, as the compiler can still "clobber" RAX and
thus load garbage into *hv_status if the hypercall faults (or if KVM
doesn't set RAX).

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vipin Sharma <vipinsh@google.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20220922062451.2927010-1-vipinsh@google.com
[sean: move to separate patch, massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/x86_64/hyperv_features.c

@@ -26,7 +26,8 @@ static inline uint8_t hypercall(u64 control, vm_vaddr_t input_address,
 		     : "=a" (*hv_status),
 		       "+c" (control), "+d" (input_address),
 		       KVM_ASM_SAFE_OUTPUTS(vector)
-		     : [output_address] "r"(output_address)
+		     : [output_address] "r"(output_address),
+		       "a" (-EFAULT)
 		     : "cc", "memory", "r8", KVM_ASM_SAFE_CLOBBERS);
 	return vector;
 }