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

[Edgecombe, Rick P]

On Tue, 2026-09-01 at 18:54 +0800, 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. 
> 


> Today only the caller
> of TDH.VP.INIT does this hack, but future kernel changes will need to
> select versions for more SEAMCALL leafs, so a less hacky solution is
> needed.

Only nit is that this would probably read a little bit smoother as two
sentences. Everything else looks good.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>

> 
> Explicitly define the arguments for seamcall*() wrappers:
> 
>   - Add a build-time assertion to ensure "fn" only contains valid
>     SEAMCALL leaf number bits. Note the P-SEAMLDR selector bit (bit 63)
>     is part of the leaf number for SEAM loader calls, so it is allowed.
>     This also means the "fn" can't be a narrower type, such as u16, to
>     exclude unrelated bits.
> 
>   - Add a "version" field in struct tdx_module_args [1], so most
>     existing callers get a default "version == 0" behavior without code
>     churn. Update the TDH.VP.INIT caller to specify the version
>     descriptively.
> 
> Encode the leaf number and tdx_module_args.version into RAX in assembly,
> because this is the place where the "fn" and struct tdx_module_args
> fields are marshaled into registers.
> 
> AI was used under supervision to review code and workshop logs. In
> particular, it helped evaluate the assembly changes.
> 
> Signed-off-by: Xu Yilun <yilun.xu@xxxxxxxxxxxxxxx>
> Reviewed-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> Reviewed-by: Nikolay Borisov <nik.borisov@xxxxxxxx>
> Link: https://lore.kernel.org/kvm/4f4b0f29-424b-45ed-8cfd-c77da2ea390f@xxxxxxxxx/ # [1]