Re: [PATCH 3/3] KVM: VMX: Initialize TDX during KVM module load

[Dan Williams <dan.j.williams@intel.com>]

Huang, Kai wrote:
> On Wed, 2024-10-30 at 08:19 -0700, Sean Christopherson wrote:
> > On Tue, Oct 29, 2024, Kai Huang wrote:
> > > diff --git a/arch/x86/kvm/Makefile b/arch/x86/kvm/Makefile
> > > index f9dddb8cb466..fec803aff7ad 100644
> > > --- a/arch/x86/kvm/Makefile
> > > +++ b/arch/x86/kvm/Makefile
> > > @@ -20,6 +20,7 @@ kvm-intel-y		+= vmx/vmx.o vmx/vmenter.o vmx/pmu_intel.o vmx/vmcs12.o \
> > >  
> > >  kvm-intel-$(CONFIG_X86_SGX_KVM)	+= vmx/sgx.o
> > >  kvm-intel-$(CONFIG_KVM_HYPERV)	+= vmx/hyperv.o vmx/hyperv_evmcs.o
> > > +kvm-intel-$(CONFIG_INTEL_TDX_HOST)	+= vmx/tdx.o
> > 
> > IMO, INTEL_TDX_HOST should be a KVM Kconfig, e.g. KVM_INTEL_TDX.  Forcing the user
> > to bounce between KVM's menu and the generic menu to enable KVM support for TDX is
> > kludgy.  Having INTEL_TDX_HOST exist before KVM support came along made sense, as
> > it allowed compile-testing a bunch of code, but I don't think it should be the end
> > state.
> > 
> > If others disagree, then we should adjust KVM_AMD_SEV in the opposite direction,
> > because doing different things for SEV vs. TDX is confusing and messy.
> 
> + Dave (and Dan for TDX Connect).
> 
> Agree SEV/TDX should be in similar way.  But also I find SEV has a dependency on
> CRYPTO_DEV_SP_PSP, so perhaps it also reasonable to make an additional
> KVM_INTEL_TDX and make it depend on INTEL_TDX_HOST?
> 
> We could remove INTEL_TDX_HOST but only keep KVM_INTEL_TDX.  But in the long
> term, more kernel components will need to add TDX support (e.g., for TDX
> Connect).  I think the question is whether we can safely disable TDX code in ALL
> kernel components when KVM_INTEL_TDX is not enabled.
> 
> If the answer is yes (seems correct to me, because it seems meaningless to
> enable TDX code in _ANY_ kernel components when it's even possible to run TDX 
> guest), then I think we can just change the current INTEL_TDX_HOST to
> KVM_INTEL_TDX and put it in arch/x86/kvm/Kconfig.

I agree with Sean's later reply that kvm-intel.ko should fail if
anything that is expected to be there and not otherwise permanently
disabled fails setup.

However, I want to provide a counterpoint to this "_ANY_ kernel
component" dependency on being able to run a TDX guest. TDX Connect like
SEV-TIO offers device-security provisioning flows that are expected to
run before any confidential guest is being launched, and theoretically
may offer services independent of *ever* launching a guest (e.g. PCIe
link encrcyption without device assignment). So longer term, seamcalls
without kvm-intel.ko flexibility is useful, but in the near term a
coarse dependency on kvm-intel.ko is workable.