Re: [RFC PATCH 3/4] KVM: TDX: Exit to userspace for GetTdVmCallInfo

[Edgecombe, Rick P]

On Tue, 2025-06-10 at 09:50 -0700, Rick Edgecombe wrote:
> Why do we need an opt-in interface instead of a way to expose which exit's are
> supported by KVM? I would think the need for a TDVMCALL opt-in interface would
> only come up if there was a bad guest that was making TDVMCALLs that it did not
> see in GetTdVmCallInfo. So that we would actually require an opt-in is not
> guaranteed. 
> 
> Another consideration could be how to handle GetQuote for an eventual TDVMCALL
> opt-in interface, should it be needed. The problem would be GetQuote would be
> opted in by default and make the interface weird. But we may not want to have a
> TDVMCall specific opt-in interface. There could be other TDX behaviors that we
> need to opt-in around. In which case the opt-in interface could be more generic,
> and by implementing the TDVMCall opt-in interface ahead of time we would end up
> with two opt-in interfaces instead of one.
> 
> So how about just adding a field to struct kvm_tdx_capabilities to describe the
> KVM TDVMcalls? Or some other place? But don't invent an opt-in interface
> until/if we need it.

Oh, and there already is a hypercall exit opt-in interface, so
KVM_CAP_TDX_USER_EXIT_TDVMCALL would overlap with it, right?