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

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Message-ID: <68758e893367f_38ba712947a@iweiny-mobl.notmuch>
Date: Mon, 14 Jul 2025 18:11:05 -0500
From: Ira Weiny <ira.weiny@...el.com>
To: Vishal Annapurve <vannapurve@...gle.com>, Sean Christopherson
	<seanjc@...gle.com>
CC: Ira Weiny <ira.weiny@...el.com>, 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>, <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()

Vishal Annapurve wrote:
> On Fri, Jul 11, 2025 at 3:56 PM Sean Christopherson <seanjc@...gle.com> 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.  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. :-)
> 
> If this is a huge page backing, starting as shared will split all the
> pages to 4K granularity upon allocation.

Why?  What is the reason it needs to be split?

> To avoid splitting, userspace
> can start with everything as private when working with hugepages and
> then follow convert to shared -> populate -> convert to private as
> needed.

I'm not saying this could not be done but seems wasteful.

Ira