Re: [PATCH v11 018/113] KVM: TDX: create/destroy VM structure

[Sean Christopherson]

On Thu, Jan 19, 2023, Huang, Kai wrote:
> On Thu, 2023-01-19 at 21:36 +0000, Sean Christopherson wrote:
> > The least invasive idea I have is expand the TDP MMU's concept of "frozen" SPTEs
> > and freeze (a.k.a. lock) the SPTE (KVM's mirror) until the corresponding S-EPT
> > update completes.
> 
> This will introduce another "having-to-wait while SPTE is frozen" problem I
> think, which IIUC means (one way is) you have to do some loop and retry, perhaps
> similar to yield_safe.

Yes, but because the TDP MMU already freezes SPTEs (just for a shorter duration),
I'm 99% sure all of the affected flows already know how to yield/bail when necessary.

The problem with the zero-step mitigation is that it could (theoretically) cause
a "busy" error on literally any accesses, which makes it infeasible for KVM to have
sane behavior.  E.g. freezing SPTEs to avoid the ordering issues isn't necessary
when holding mmu_lock for write, whereas the zero-step madness brings everything
into play.