Re: [PATCH v3 09/11] KVM: arm64: Introduce KVM_VM_TYPE_ARM_SW_PROTECTED machine type

[Quentin Perret <qperret@google.com>]

On Tuesday 11 Feb 2025 at 17:09:20 (+0000), Quentin Perret wrote:
> Hi Patrick,
> 
> On Tuesday 11 Feb 2025 at 16:32:31 (+0000), Patrick Roy wrote:
> > I was hoping that SW_PROTECTED_VM will be the VM type that something
> > like Firecracker could use, e.g. an interface to guest_memfd specifically
> > _without_ pKVM, as Fuad was saying.
> 
> I had, probably incorrectly, assumed that we'd eventually want to allow
> gmem for all VMs, including traditional KVM VMs that don't have anything
> special. Perhaps the gmem support could be exposed via a KVM_CAP in this
> case?
> 
> Anyway, no objection to the proposed approach in this patch assuming we
> will eventually have HW_PROTECTED_VM for pKVM VMs, and that _that_ can be
> bit 31 :).

Thinking about this a bit deeper, I am still wondering what this new
SW_PROTECTED VM type is buying us? Given that SW_PROTECTED VMs accept
both guest-memfd backed memslots and traditional HVA-backed memslots, we
could just make normal KVM guests accept guest-memfd memslots and get
the same thing? Is there any reason not to do that instead? Even though
SW_PROTECTED VMs are documented as 'unstable', the reality is this is
UAPI and you can bet it will end up being relied upon, so I would prefer
to have a solid reason for introducing this new VM type.

Cheers,
Quentin