Re: [RFC PATCH 02/15] x86/tdx: Add TDCM hypercall wrapper for TDX Connect

[Nikolay Borisov]

On 14.10.26 г. 9:19 ч., Duan, Zhenzhong wrote:
> > -----Original Message-----
> > From: Nikolay Borisov <nik.borisov@xxxxxxxx>
> > Subject: Re: [RFC PATCH 02/15] x86/tdx: Add TDCM hypercall wrapper for TDX
> > Connect
> >
> >
> >
> > On 24.09.26 г. 7:10 ч., Zhenzhong Duan wrote:
> > > TDX Connect is a TDX feature that enables secure device assignment,
> > > allowing a TDX guest to directly interact with TEE-IO capable devices.
> > >
> > > The TEE-IO Device Control and Management (TDCM) hypercall
> > > (TDG.VP.VMCALL<TDCM>, defined in TDX GHCI v2.0 specification) is the
> > > primary interface through which a TDX guest issues device management
> > > commands to the host. The host processes these commands and returns
> > > responses via a shared memory buffer.
> >
> > This patch contradicts what's going in patch 5. Because you make it
> > sound as if TDCM is yet another leaf of VP.VMCALL, but then in patch 5
> > you introduce the polling code to read the result. And there is not
> > enough information in the changelogs to understand why this added
> > complexity is needed. Isn't TDCM handled synchronously by the VMM as the
> > rest of the tdcalls, is it perhaps handled by TDX NRX module?
>
> TDG.VP.VMCALL<TDCM> is the leaf used to submit a TDCM request. The VMM’s
> return from that VMCALL acknowledges the request; it does not
> necessarily mean the device-management operation has completed. GHCI
> defines asynchronous completion for potentially long-running operations.

But this is essentially an implementation detail, and currently we don't 
have the host implementation. In that case this choice must be spelled out.

> GHCI describes polling the shared status and event notification as
> completion mechanisms. This series uses polling with a timeout; it does
> not use event notification. TDCM is not implemented by the NRX module,
> although some host-side operations initiated by TDCM may use NRX
> internally.

Right, those operation falls within the TDX connect spec. And TDCM is 
just a subset of the VP.VMCALL interface used for tdx connect 
management, in that sense it's not a "separate entity" i.e a discrete 
module but just a collection of functionality, implemented in the VMM?

> I’ll clarify the submission-versus-completion distinction in this
> patch’s changelog and explain the polling choice in patch 5.
>
> Thanks
> Zhenzhong