linux-kernel - Re: [RFC PATCH] KVM: TDX: Decouple TDX init mem region from kvm_gmem

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Message-ID: <aHGWtsqr8c403nIj@google.com>
Date: Fri, 11 Jul 2025 15:56:54 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Ira Weiny <ira.weiny@...el.com>
Cc: Michael Roth <michael.roth@....com>, Yan Zhao <yan.y.zhao@...el.com>, pbonzini@...hat.com, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, rick.p.edgecombe@...el.com, 
	kai.huang@...el.com, adrian.hunter@...el.com, reinette.chatre@...el.com, 
	xiaoyao.li@...el.com, tony.lindgren@...el.com, binbin.wu@...ux.intel.com, 
	dmatlack@...gle.com, isaku.yamahata@...el.com, vannapurve@...gle.com, 
	david@...hat.com, ackerleytng@...gle.com, tabba@...gle.com, 
	chao.p.peng@...el.com
Subject: Re: [RFC PATCH] KVM: TDX: Decouple TDX init mem region from kvm_gmem_populate()

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.  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. :-)

> 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.