Re: [PATCH v12 20/23] KVM: selftests: Add support for TDX TDCALL from guest

[Sean Christopherson <seanjc@google.com>]

On Fri, Oct 31, 2025, Sean Christopherson wrote:
> On Fri, Oct 31, 2025, Ira Weiny wrote:
> > Sagi Shahar wrote:
> > > From: Erdem Aktas <erdemaktas@google.com>
> > > 
> > > Add support for TDX guests to issue TDCALLs to the TDX module.
> > 
> > Generally it is nice to have more details.  As someone new to TDX I
> > have to remind myself what a TDCALL is.  And any random kernel developer
> > reading this in the future will likely have even less clue than me.
> > 
> > Paraphrased from the spec:
> > 
> > TDCALL is the instruction used by the guest TD software (in TDX non-root
> > mode) to invoke guest-side TDX functions.  TDG.VP.VMCALL helps invoke
> > services from the host VMM.
> > 
> > Add support for TDX guests to invoke services from the host VMM.
> 
> Eh, at some point a baseline amount of knowledge is required.  I highly doubt
> regurgitating the spec is going to make a huge difference
> 
> I also dislike the above wording, because it doesn't help understand _why_ KVM
> selftests need to support TDCALL, or _how_ the functionality will be utilized.
> E.g. strictly speaking, we could write KVM selftests without ever doing a single
> TDG.VP.VMCALL, because we control both sides (guest and VMM).  And I have a hard
> time belive name-dropping TDG.VP.VMCALL is going to connect the dots between
> TDCALL and the "tunneling" scheme defined by the GHCI for requesting emulation
> of "legacy" functionality".
> 
> What I would like to know is why selftests are copy-pasting the kernel's scheme
> for marshalling data to/from the registers used by TDCALL, 

I almost forgot.  I detest the "throw everything into a structure" approach,
which the kernel used largely so that it could share code between SEAMCALLs and
TDCALLs.  Unless there's a good reason no to, I would much rather have prototypes
like

  uint64_t __tdvmcall(<all the args>)
  uint64_t tdvmcall_1(uint64_t arg1);
  uint64_t tdvmcall_2(uint64_t arg1, uint64_t arg2);
  uint64_t tdvmcall_3(...);
  uint65_t tdvmcall_4(...);
  uint64_t tdvmcall_5(...);
  uint64_t tdvmcall_6(...);