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

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Message-ID: <68758e01d3ae4_38ba7129493@iweiny-mobl.notmuch>
Date: Mon, 14 Jul 2025 18:08:49 -0500
From: Ira Weiny <ira.weiny@...el.com>
To: Sean Christopherson <seanjc@...gle.com>, 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()

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