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Date: Wed, 9 Jul 2025 08:00:04 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Vishal Annapurve <vannapurve@...gle.com>
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Subject: Re: [RFC PATCH v2 00/51] 1G page support for guest_memfd

On Wed, Jul 09, 2025, Vishal Annapurve wrote:
> I think we can simplify the role of guest_memfd in line with discussion [1]:

I genuinely don't understand what you're trying to "simplify".  We need to define
an ABI that is flexible and robust, but beyond that most of these guidelines boil
down to "don't write bad code".

> 1) guest_memfd is a memory provider for userspace, KVM, IOMMU.

No, guest_memfd is a memory provider for KVM guests.  That memory *might* be
mapped by userspace and/or into IOMMU page tables in order out of functional
necessity, but guest_memfd exists solely to serve memory to KVM guests, full stop.

> 3) KVM should ideally associate the lifetime of backing
> pagetables/protection tables/RMP tables with the lifetime of the
> binding of memslots with guest_memfd.

Again, please align your indentation.

>          - Today KVM SNP logic ties RMP table entry lifetimes with how
>            long the folios are mapped in guest_memfd, which I think should be
>            revisited.

Why?  Memslots are ephemeral per-"struct kvm" mappings.  RMP entries and guest_memfd
inodes are tied to the Virtual Machine, not to the "struct kvm" instance.

> Some very early thoughts on how guest_memfd could be laid out for the long term:
> 1) guest_memfd code ideally should be built-in to the kernel.

Why?  How is this at all relevant?  If we need to bake some parts of guest_memfd
into the kernel in order to avoid nasty exports and/or ordering dependencies, then
we can do so.  But that is 100% an implementation detail and in no way a design
goal.

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