Re: [RFC PATCH 09/21] KVM: TDX: Enable 2MB mapping size after TD is RUNNABLE

[Sean Christopherson]

On Thu, Jun 12, 2025, Rick P Edgecombe wrote:
> On Wed, 2025-06-11 at 07:42 -0700, Sean Christopherson wrote:
> > If there's a *legitimate* use case where the guest wants to ACCEPT a subset of
> > memory, then there should be an explicit TDCALL to request that the unwanted
> > regions of memory be unmapped.  Smushing everything into implicit behavior has
> > obvioulsy created a giant mess.
> 
> Hi, still digging on if there is any possible use.
> 
> I think this may need a guest opt-in, so the guest can say it can handle errors
> for both smaller and larger page size matches. So it may not matter if there is
> a rare usage or not. If KVM finds the guest opts-in (how to do that TBD), it can
> start mapping at the host level. 

Hmm, clever.  That should work; requiring an updated guest kernel to get optimal
performance doesn't seem too onerous.

> If KVM doesn't see the opt-in, the guest gets 4k pages.

As in, KVM doesn't even try to use hugepage mappings?  If so, this idea probably
gets my vote.