Re: [PATCH v2] x86/virt/tdx: Formalize SEAMCALL version encoding support

[Kiryl Shutsemau <kas@kernel.org>]

On Thu, Jul 09, 2026 at 01:03:30AM +0800, Xu Yilun wrote:
> SEAMCALL invokes TDX module functions using a function number and
> parameters. To extend the functionalities of existing SEAMCALLs while
> keeping backward compatibility, TDX adds more numbered SEAMCALLs of the
> same family. This is just like syscalls, except that TDX defines a
> specific function number encoding pattern: a base function number and a
> version together encode the full function number.
> 
> An existing SEAMCALL helper (TDH.VP.INIT) is already using the version
> field. Having the caller pack the version into the function number
> open-codes the ABI layout.
> 
> Add a version field in struct tdx_module_args [1], so that most existing
> SEAMCALL helpers get a default "version == 0" behavior without code
> churn, while callers requiring extended functionalities can specify the
> version descriptively. Encode the tdx_module_args.version in the
> function number before calling into assembly code.

The commit message covers the mechanics, but is silent on compatibility
with older TDX modules. That context matters for the design choice, so
regardless of where the implementation discussion lands, I think it
needs to be spelled out here.

Who is responsible for picking a version the module supports?

TDH.VP.INIT can hardcode version 1 only because KVM already refuses to
enable TDX on modules without TOPOLOGY_ENUM.

The TDH.SYS.UPDATE user from [1] is the opposite case: it has to pick
version 0 or 1 at runtime depending on whether add-on features are
configured, to keep working on modules that don't support them.

The second point is the actual argument for a version field in struct
tdx_module_args rather than encoding the version in the leaf defines:
the version is not always a compile-time property of the call site.

Without TDH.SYS.UPDATE context, the patch seems pointless.

> Link: https://lore.kernel.org/kvm/4f4b0f29-424b-45ed-8cfd-c77da2ea390f@intel.com/ # [1]

-- 
  Kiryl Shutsemau / Kirill A. Shutemov