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

[Sean Christopherson <seanjc@google.com>]

On Thu, Oct 31, 2024, Kai Huang wrote:
> On Wed, 2024-10-30 at 08:19 -0700, Sean Christopherson wrote:
> > > +void __init tdx_bringup(void)
> > > +{
> > > +	enable_tdx = enable_tdx && !__tdx_bringup();
> > 
> > Ah.  I don't love this approach because it mixes "failure" due to an unsupported
> > configuration, with failure due to unexpected issues.  E.g. if enabling virtualization
> > fails, loading KVM-the-module absolutely should fail too, not simply disable TDX.
> 
> Thanks for the comments.
> 
> I see your point.  However for "enabling virtualization failure" kvm_init() will
> also try to do (default behaviour), so if it fails it will result in module
> loading failure eventually.  So while I guess it would be slightly better to
> make module loading fail if "enabling virtualization fails" in TDX, it is a nit
> issue to me.
> 
> I think "enabling virtualization failure" is the only "unexpected issue" that
> should result in module loading failure.  For any other TDX-specific
> initialization failure (e.g., any memory allocation in future patches) it's
> better to only disable TDX.

I disagree.  The platform owner wants TDX to be enabled, KVM shouldn't silently
disable TDX because of a transient, unrelated failure.

If TDX _can't_ be supported, e.g. because EPT or MMIO SPTE caching was explicitly
disable, then that's different.  And that's the general pattern throughout KVM.
If a requested feature isn't supported, then KVM continues on updates the module
param accordingly.  But if something outright fails during setup, KVM aborts the
entire sequence.

> So I can change to "make loading KVM-the-module fail if enabling virtualization
> fails in TDX", but I want to confirm this is what you want?

I would prefer the logic to be: reject loading kvm-intel.ko if an operation that
would normally succeed, fails.