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

Sean Christopherson
2024-04-30T16:13:30Z

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.