Re: [PATCH v5] x86/virt/tdx: Formalize SEAMCALL leaf version encoding support

[Binbin Wu]

On 9/1/2026 6:54 PM, Xu Yilun wrote:
> The TDX architecture includes a syscall-like ABI for OSes to communicate
> with SEAM mode software. This ABI has the concept of a "leaf". Each leaf
> is roughly analogous to a Linux syscall: it has a set of register
> arguments and does one logical thing like adding a page of memory to a
> VM or running a VM. The TDX architecture refers to this interface
> function as a "SEAMCALL leaf".
> 
> Just like syscalls, the TDX architecture wants to evolve the ABI to
> extend functionality while keeping compatibility. But unlike syscalls,
> which do this by picking a totally new syscall number, the ABI encodes
> the "version" number directly into the bits of a register argument. So
> instead of openatN being whatever the next free number is, the SEAMCALL
> leaf number and version number are encoded into certain bits in how the
> RAX register is defined in the ABI.
> 
> In Linux, several seamcall*() wrappers have been introduced to invoke
> SEAMCALL leafs. They all take a u64 "fn" argument for the leaf number
> which eventually get set in the register. As above, the version number
> lives in the same register as the leaf number. So callers that want to
> select a specific version of the leaf, can jam it in the right place in
> the register by passing it in the "fn" argument.

Do you think it's worth explaining how callers determine the versions
supported by the TDX module, and what happens if the specified version
is not supported by the TDX module?