Re: Rename restrictedmem => guardedmem? (was: Re: [PATCH v10 0/9] KVM: mm: fd-based approach for supporting KVM)

[David Hildenbrand]

On 19.04.23 02:47, Sean Christopherson wrote:
> On Tue, Apr 18, 2023, David Hildenbrand wrote:
> > On 17.04.23 21:16, Sean Christopherson wrote:
> > > Hidden/Concealed/etc - Too close to secretmem, suffers the "hidden from whom" problem,
> > > and depending on the use case, the memory may not actually be concealed from the
> > > user that controls the VMM.
> > >
> > > Restricted - "rmem" collides with "reserved memory" in code.
> > >
> > > Guarded - Conflicts with s390's "guarded storage", has the "from whom" problem.
> > >
> > > Inaccessible - Many of the same problems as "hidden".
> > >
> > > Unmappable - Doesn't cover things like read/write, and is wrong in the sense that
> > > the memory is still mappable, just not via mmap().
> > >
> > > Secured - I'm not getting anywhere near this one :-)
> >
> > The think about "secretmem" that I kind-of like (a little) is that it
> > explains what it's used for: storing secrets. We don't call it "unmapped"
> > memory because we unmap it from the directmap or "unpinnable" memory or
> > "inaccessible" memory ... or even "restricted" because it has restrictions
> > ... how the secrets are protected is kind of an implementation detail.
> >
> > So instead of describing *why*/*how* restrictedmem is the weird kid
> > (restricted/guarded/hidden/restricted/inaccessible/ ...), maybe rather
> > describe what it's used for?
> >
> > I know, I know, "there are other use cases where it will be used outside of
> > VM context". I really don't care.
>
> Heh, we originally proposed F_SEAL_GUEST, but that was also sub-optimal[1] ;-)
>
> > "memfd_vm" / "vm_mem" would be sooo (feel free to add some more o's here)
> > much easier to get. It's a special fd to be used to back VM memory. Depending
> > on the VM type (encrypted/protected/whatever), restrictions might apply (not
> > able to mmap, not able to read/write ...). For example, there really is no
> > need to disallow mmap/read/write when using that memory to back a simple VM
> > where all we want to do is avoid user-space page tables.
>
> In seriousness, I do agree with Jason's very explicit objection[2] against naming
> a non-KVM uAPI "guest", or any variation thereof.

While I agree, it's all better than the naming we use right now ...


Let me throw "tee_mem" / "memfd_tee" into the picture. That could 
eventually catch what we want to have.

Or "coco_mem" / "memfd_coco".

Of course, both expect that people know the terminology (just like what 
"vm" stands for), but it's IMHO significantly better than 
restricted/guarded/opaque/whatsoever.

Again, expresses what it's used for, not why it behaves in weird ways.


> An alternative that we haven't considered since the very early days is making the
> uAPI a KVM ioctl() instead of a memfd() flag or dedicated syscall.  Looking at the
> code for "pure shim" implementation[3], that's actually not that crazy of an idea.

Yes.

> I don't know that I love the idea of burying this in KVM, but there are benefits
> to coupling restrictedmem to KVM (aside from getting out from behind this bikeshed
> that I created).

Yes, it's all better than jumping through hoops to come up with a bad 
name like "restrictedmem".

> The big benefit is that the layer of indirection goes away.  That simplifies things
> like enhancing restrictedmem to allow host userspace access for debug purposes,
> batching TLB flushes if a PUNCH_HOLE spans multiple memslots, enforcing exclusive
> access, likely the whole "share with a device" story if/when we get there, etc.
>
> The obvious downsides are that (a) maintenance falls under the KVM umbrella, but
> that's likely to be true in practice regardless of where the code lands, and

Yes.

> (b) if another use case comes along, e.g. the Gunyah hypervisor[4][5], we risk
> someone reinventing a similar solution.

I agree. But if it's as simple as providing an ioctl for that hypervisor 
that simply wires up the existing implementation, it's not too bad.

> If we can get Gunyah on board and they don't need substantial changes to the
> restrictedmem implementation, then I'm all for continuing on the path we're on.
> But if Gunyah wants to do their own thing, and the lightweight shim approach is
> viable, then it's awfully tempting to put this all behind a KVM ioctl().

Right. Or we still succeed in finding a name that's not as bad as what 
we had so far.

--
Thanks,

David / dhildenb