KVM: TDX: Add new TDVMCALL status code for unsupported subfuncs

Add the new TDVMCALL status code TDVMCALL_STATUS_SUBFUNC_UNSUPPORTED and
return it for unimplemented TDVMCALL subfunctions.

Returning TDVMCALL_STATUS_INVALID_OPERAND when a subfunction is not
implemented is vague because TDX guests can't tell the error is due to
the subfunction is not supported or an invalid input of the subfunction.
New GHCI spec adds TDVMCALL_STATUS_SUBFUNC_UNSUPPORTED to avoid the
ambiguity. Use it instead of TDVMCALL_STATUS_INVALID_OPERAND.

Before the change, for common guest implementations, when a TDX guest
receives TDVMCALL_STATUS_INVALID_OPERAND, it has two cases:
1. Some operand is invalid. It could change the operand to another value
retry.
2. The subfunction is not supported.

For case 1, an invalid operand usually means the guest implementation bug.
Since the TDX guest can't tell which case is, the best practice for
handling TDVMCALL_STATUS_INVALID_OPERAND is stopping calling such leaf,
treating the failure as fatal if the TDVMCALL is essential or ignoring
it if the TDVMCALL is optional.

With this change, TDVMCALL_STATUS_SUBFUNC_UNSUPPORTED could be sent to
old TDX guest that do not know about it, but it is expected that the
guest will make the same action as TDVMCALL_STATUS_INVALID_OPERAND.
Currently, no known TDX guest checks TDVMCALL_STATUS_INVALID_OPERAND
specifically; for example Linux just checks for success.

Signed-off-by: Binbin Wu <[e-mail]>
[Return it for untrapped KVM_HC_MAP_GPA_RANGE. - Paolo]
Signed-off-by: Paolo Bonzini <[e-mail]>

arch/x86/include/asm/shared/tdx.h

···
 #define TDVMCALL_STATUS_RETRY		0x0000000000000001ULL
 #define TDVMCALL_STATUS_INVALID_OPERAND	0x8000000000000000ULL
 #define TDVMCALL_STATUS_ALIGN_ERROR	0x8000000000000002ULL
+#define TDVMCALL_STATUS_SUBFUNC_UNSUPPORTED	0x8000000000000003ULL
 
 /*
  * Bitmasks of exposed registers (with VMM).

arch/x86/kvm/vmx/tdx.c

···
 	/*
 	 * Converting TDVMCALL_MAP_GPA to KVM_HC_MAP_GPA_RANGE requires
 	 * userspace to enable KVM_CAP_EXIT_HYPERCALL with KVM_HC_MAP_GPA_RANGE
-	 * bit set.  If not, the error code is not defined in GHCI for TDX, use
-	 * TDVMCALL_STATUS_INVALID_OPERAND for this case.
+	 * bit set.  This is a base call so it should always be supported, but
+	 * KVM has no way to ensure that userspace implements the GHCI correctly.
+	 * So if KVM_HC_MAP_GPA_RANGE does not cause a VMEXIT, return an error
+	 * to the guest.
 	 */
 	if (!user_exit_on_hypercall(vcpu->kvm, KVM_HC_MAP_GPA_RANGE)) {
-		ret = TDVMCALL_STATUS_INVALID_OPERAND;
+		ret = TDVMCALL_STATUS_SUBFUNC_UNSUPPORTED;
 		goto error;
 	}
 
···
 		break;
 	}
 
-	tdvmcall_set_return_code(vcpu, TDVMCALL_STATUS_INVALID_OPERAND);
+	tdvmcall_set_return_code(vcpu, TDVMCALL_STATUS_SUBFUNC_UNSUPPORTED);
 	return 1;
 }