Re: [Invitation] bi-weekly guest_memfd upstream call on 2024-12-12

["Gowans, James" <jgowans@amazon.com>]

On Tue, 2024-12-10 at 15:25 +0100, David Hildenbrand wrote:
> In this meeting we'll likely discuss:
>   * Patrick: KVM gmem MMIO access challenges and KVM_X86_SW_PROTECTED_VM
>     for arm
>   * Aneesh: Feasibility of 4 KiB guests on 64 KiB host
>   * Persisting guest_memfd across reboot / guest_memfs (if James is around)

I should be around and will be keen to discuss this. Where we landed
last on this topic was that guestmemfs should be modified to use the
guest_memfd library code to instantiate a real guest_memfd file when a
guestmemfs file is opened. Essentially the guestmemfs persistence will
mostly be a custom allocator behind the in-development guest_memfd
library code, including the ability to restore guest_memfd mappings when
re-opening the file after kexec.

The main dependency here is on the guest_memfd library effort.
Discussion on how that's going and making sure that the guestmemfs
persistence use case is covered will be useful. 

We need to make sure that the guest_memfd library supports:
1. Defining a custom allocator other than buddy-list managed pages so
that a persistent reserved memory pool can be used.
2. Being able to re-drive or fault in mappings for a file after kexec.
In all likelihood the allocator code path and equally restore previously
allocated pages.
3. Support huge/gigantic mappings
4. Support mmaping the guest_memfd file; for non-CoCo VMs this will be
necessary for PV devices. I realise that this is a whole can of worms
currently under discussion and not specific to this use case.

So, let's socialise this revised guestmemfs approach, and next steps for
the library development.

> 
> And we'll continue our discussion on:
>   * Challenges with supporting huge pages
>   * Challenges with shared vs. private conversion
>   * guest_memfd as a "library"


JG



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