[RFC PATCH 2/3] KVM: arm64: Allow userspace to trap SMCCC sub-ranges

[Marc Zyngier]

On Fri, 11 Nov 2022 23:39:09 +0000,
Oliver Upton <oliver.upton at linux.dev> wrote:
> 
> On Fri, Nov 11, 2022 at 08:26:02AM +0000, Marc Zyngier wrote:
> > On Thu, 10 Nov 2022 21:13:54 +0000, Oliver Upton <oliver.upton at linux.dev> wrote:
> > > The goal of what I was trying to get at is that either the kernel or
> > > userspace takes ownership of a range that has an ABI, but not both. i.e.
> > > you really wouldn't want some VMM or cloud provider trapping portions of
> > > KVM's vendor-specific range while still reporting a 'vanilla' ABI at the
> > > time of discovery. Same goes for PSCI, TRNG, etc.
> > 
> > But I definitely think this is one of the major use cases. For
> > example, there is value in taking PSCI to userspace in order to
> > implement a newer version of the spec, or to support sub-features that
> > KVM doesn't (want to) implement. I don't think this changes the ABI from
> > the guest perspective.
> 
> I disagree for the implications of partially trapping the 'Vendor
> Specific Hypervisor Service'. If the UID for the range still reports KVM
> but userspace decided to add some new widget, then from the guest
> perspective that widget is now part of KVM's own ABI with the guest.

But that's what I mean by "I don't think this changes the ABI from the
guest perspective". The guest cannot know who is doing the emulation
anyway, so it is userspace's duty to preserve the illusion. At the
end of the day, this is only a configuration mechanism, and it is no
different from all other configuration bits (i.e. they need to be
identical on both side for migration).

> Trapping the whole range is a bit of a hack to workaround the need for
> more complicated verification of a hypercall filter.

We already need these things for architected hypercalls. Once we have
the infrastructure, it doesn't matter anymore which range this is for.

> 
> But for everything else, I'm fine with arbitrary function filtering.
> Userspace is always welcome to shoot itself in the foot.
> 
> > pKVM also has a use case for this where userspace gets a notification
> > of the hypercall that a guest has performed to share memory.
> 
> Is that hypercall in the 'Vendor Specific Hypervisor Service' range?

Yes. It is get another KVM hypercall.

> 
> > Communication with a TEE also is on the cards, as would be a FFA
> > implementation. All of this could be implemented in KVM, or in
> > userspace, depending what users of these misfeatures want to do.
> 
> I'm very hopeful that by forwarding all of this to userspace we can get
> out of the business of implementing every darn spec that comes along.

Good luck. All the TEEs have private, home grown APIs, and every
vendor will want to implement their own crap (i.e. there is no spec).

	M.

-- 
Without deviation from the norm, progress is not possible.