[RFC PATCH 1/4] KVM: TDX: Add new TDVMCALL status code for unsupported subfuncs

[Binbin Wu <binbin.wu@linux.intel.com>]

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.
Treat it as fatal if the TDVMCALL is essential or ignore it if the TDVMCALL
is optional.

After the change, TDVMCALL_STATUS_SUBFUNC_UNSUPPORTED is unknown to the
old TDX guest, the guest will make the same action as
TDVMCALL_STATUS_INVALID_OPERAND, unless the guest check the
TDVMCALL_STATUS_INVALID_OPERAND specifically.  Currently, no known
TDX guests do it, e.g., Linux TDX guests just check for success.

Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
---
 arch/x86/include/asm/shared/tdx.h | 1 +
 arch/x86/kvm/vmx/tdx.c            | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/shared/tdx.h b/arch/x86/include/asm/shared/tdx.h
index fd9209e996e7..b109b947fadf 100644
--- a/arch/x86/include/asm/shared/tdx.h
+++ b/arch/x86/include/asm/shared/tdx.h
@@ -79,6 +79,7 @@
 #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).
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index b952bc673271..8134d5805b03 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1476,7 +1476,7 @@ static int handle_tdvmcall(struct kvm_vcpu *vcpu)
 		break;
 	}
 
-	tdvmcall_set_return_code(vcpu, TDVMCALL_STATUS_INVALID_OPERAND);
+	tdvmcall_set_return_code(vcpu, TDVMCALL_STATUS_SUBFUNC_UNSUPPORTED);
 	return 1;
 }
 
-- 
2.46.0