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

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Message-ID: <aHWOx6-IztKB--Af@google.com>
Date: Mon, 14 Jul 2025 16:12:07 -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 Mon, Jul 14, 2025, Ira Weiny wrote:
> 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.

IIUC, passing data out is supported and used by pKVM.  I can see use cases for
things where the output of some processing is a non-trivial amount of data.
E.g. pass in "public" data/inputs, protect the data in-place, process the data
using a super secret model, then finally pass the sanitized, "safe for public
consumption" result back to the untrusted world.