Re: [RFC PATCH v3 47/59] KVM: TDX: Define TDCALL exit reason

Sean Christopherson
2021-11-29T22:57:56Z

On Thu, Nov 25, 2021, Thomas Gleixner wrote:
> On Wed, Nov 24 2021 at 16:20, isaku yamahata wrote:
> > From: Sean Christopherson <sean.j.christopherson@intel.com>
> >
> > Define the TDCALL exit reason, which is carved out from the VMX exit
> > reason namespace as the TDCALL exit from TDX guest to TDX-SEAM is really
> > just a VM-Exit.
>
> How is this carved out? What's the value of this word salad?
>
> It's simply a new exit reason. Not more, not less. So what?

The changelog is alluding to the fact that KVM should never directly see a TDCALL
VM-Exit. For TDX, KVM deals only with "returns" from the TDX-Module. The "carved
out" bit is calling out that the transition from SEAM Non-Root (the TDX guest) to
SEAM Root (the TDX Module) is actually a VT-x/VMX VM-Exit, e.g. if TDX were somehow
implemented without relying on VT-x/VMX, then the TDCALL exit reason wouldn't exist.

> > Co-developed-by: Xiaoyao Li <xiaoyao.li@intel.com>
> > Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
>
> I'm pretty sure that it does not take two engineers to add a new exit
> reason define, but it takes at least two engineers to come up with a
> convoluted explanation for it.

Nah, just one ;-)