Re: [PATCH v19 023/130] KVM: TDX: Initialize the TDX module when loading the KVM intel kernel module

[Sean Christopherson]

On Tue, Apr 30, 2024, Kai Huang wrote:
> On 30/04/2024 8:06 am, Sean Christopherson wrote:
> > My suggestion is essentially "throw in a CR4.VMXE check before
> > TDH.SYS.LP.INIT if it's easy".  If it's not easy for some reason, then don't do
> > it.
> 
> I see.  The disconnection between us is I am not super clear why we should
> treat TDH.SYS.LP.INIT as a special one that deserves a CR4.VMXE check but
> not other SEAMCALLs.

Because TDH.SYS.LP.INIT is done on all CPUs via an IPI function call, is a one-
time thing, and is at the intersection of core TDX and KVM module code, e.g. the
the core TDX code has an explicit assumption that:

 * This function assumes the caller has: 1) held read lock of CPU hotplug
 * lock to prevent any new cpu from becoming online; 2) done both VMXON
 * and tdx_cpu_enable() on all online cpus.

KVM can obviously screw up and attempt SEAMCALLs without being post-VMXON, but
that's entirely a _KVM_ bug.  And the probability of getting all the way to
something like TDH_MEM_SEPT_ADD without being post-VMXON is comically low, e.g.
KVM and/or the kernel would likely crash long before that point.