Re: [RFC PATCH] KVM: TDX: Decouple TDX init mem region from kvm_gmem_populate()

[Ira Weiny <ira.weiny@intel.com>]

Sean Christopherson wrote:
> On Fri, Jul 11, 2025, Ira Weiny wrote:
> > Michael Roth wrote:
> > > For in-place conversion: the idea is that userspace will convert
> > > private->shared to update in-place, then immediately convert back
> > > shared->private;
> > 
> > Why convert from private to shared and back to private?  Userspace which
> > knows about mmap and supports it should create shared pages, mmap, write
> > data, then convert to private.
> 
> Dunno if there's a strong usecase for converting to shared *and* populating the
> data, but I also don't know that it's worth going out of our way to prevent such
> behavior, at least not without a strong reason to do so.

I'm not proposing to prevent such behavior.  Only arguing that the
private->shared->private path to data population is unlikely to be a
'common' use case.

> E.g. if it allowed for
> a cleaner implementation or better semantics, then by all means.  But I don't
> think that's true here?  Though I haven't thought hard about this, so don't
> quote me on that. :-)

Me neither.  Since I am new to this I am looking at this from a pretty
hight level and it seems to me if the intention is to pass data to the
guest then starting shared is the way to go.  Passing data out, in a Coco
VM, is probably not going to be supported.

I have to think on Vishal's assertion that a shared page needs to be split
on allocation.  That does not make sense to me.

> > Old userspace will create private and pass in a source pointer for the
> > initial data as it does today.
> > 
> > Internally, the post_populate() callback only needs to know if the data is
> > in place or coming from somewhere else (ie src != NULL).
> 
> I think there will be a third option: data needs to be zeroed, i.e. the !src &&
> !PRESERVED case.

Yes, indeed.

Ira