linux-kernel - Re: [RFC PATCH 26/39] KVM: guest_memfd: Track faultability within a struct kvm_gmem

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Message-ID: <20241017191829.GA3559746@nvidia.com>
Date: Thu, 17 Oct 2024 16:18:29 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Peter Xu <peterx@...hat.com>
Cc: David Hildenbrand <david@...hat.com>,
	Ackerley Tng <ackerleytng@...gle.com>, tabba@...gle.com,
	quic_eberman@...cinc.com, roypat@...zon.co.uk, rientjes@...gle.com,
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	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH 26/39] KVM: guest_memfd: Track faultability within a
 struct kvm_gmem_private

On Thu, Oct 17, 2024 at 03:11:10PM -0400, Peter Xu wrote:
> On Thu, Oct 17, 2024 at 02:10:10PM -0300, Jason Gunthorpe wrote:
> > > If so, maybe that's a non-issue for non-CoCo, where the VM object /
> > > gmemfd object (when created) can have a flag marking that it's
> > > always shared and can never be converted to private for any page
> > > within.
> > 
> > What is non-CoCo? Does it include the private/shared concept?
> 
> I used that to represent the possible gmemfd use cases outside confidential
> computing.
> 
> So the private/shared things should still be around as fundamental property
> of gmemfd, but it should be always shared and no convertion needed for the
> whole lifecycle of the gmemfd when marked !CoCo.

But what does private mean in this context?

Is it just like a bit of additional hypervisor security that the page
is not mapped anyplace except the KVM stage 2 and the hypervisor can
cause it to become mapped/shared at any time? But the guest has no
idea about this?

Jason