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

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Message-ID: <20241017164713.GF3559746@nvidia.com>
Date: Thu, 17 Oct 2024 13:47:13 -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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	pgonda@...gle.com, oliver.upton@...ux.dev,
	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 10:58:29AM -0400, Peter Xu wrote:

> My question was more torwards whether gmemfd could still expose the
> possibility to be used in VA forms to other modules that may not support
> fd+offsets yet.

I keep hearing they don't want to support page pinning on a guestmemfd
mapping, so VA based paths could not work.

> I think as long as we can provide gmemfd VMAs like what this series
> provides, it sounds possible to reuse the old VA interfaces before the CoCo
> interfaces are ready, so that people can already start leveraging gmemfd
> backing pages.

And you definitely can't get the private pages out of the VA interface
because all the VMA PTEs of private pages are non-present by definition.

Hence, you must use the FD for a lot of use cases here.

Jason