Re: [RFC PATCH v1 0/5] x86/boot, KVM: Move VMXON/VMXOFF handling from KVM to CPU lifecycle

Sean Christopherson
2025-09-16T17:54:48Z

On Thu, Sep 11, 2025, Arjan van de Ven wrote:
> Hi,
> > I also want to keep the code as a module, both to avoid doing VMXON unconditionally,
>
> can you expand on what the problem is with having VMXON unconditionally enabled?

Unlike say EFER.SVME, VMXON fundamentally changes CPU behavior. E.g. blocks INIT,
activates VMCS caches (which aren't cleared by VMXOFF on pre-SPR CPUs, and AFAIK
Intel hasn't even publicly committed to that behavior for SPR+), restricts allowed
CR0 and CR4 values, raises questions about ucode patch updates, triggers unique
flows in SMI/RSM, prevents Intel PT from tracing on certain CPUs, and probably a
few other things I'm forgetting.

> A lot of things are much simpler if it's on at cpu up, and turned off only at the
> down path (be it offline of kexec).. no refcounting, no locking, etc...

For Intel. Unless _all_ vendors and architectures follow suit, KVM will need
the refcounting and locking. And while it's not anyone's fault, the *vast*
majority of complexity around enabling virtualization in KVM is due to VMX.
I.e. KVM added a bunch of code to deal with the aformentioned side effects of
VMXON, and as a result, all other vendors/architectures have had to deal with
that complexity.

> so would be good to understand what the problem would be with having it always on

Doing VMXON unconditionally is a minor objection. My primary objection is that
this series does what's easiest for TDX, and leaves behind all of the VMX-induced
technical debt in KVM.