Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory

[James Bottomley <jejb@linux.ibm.com>]

On Wed, 2021-09-01 at 08:54 -0700, Andy Lutomirski wrote:
[...]
> If you want to swap a page on TDX, you can't.  Sorry, go directly to
> jail, do not collect $200.

Actually, even on SEV-ES you can't either.  You can read the encrypted
page and write it out if you want, but unless you swap it back to the
exact same physical memory location, the encryption key won't work. 
Since we don't guarantee this for swap, I think swap won't actually
work for any confidential computing environment.

> So I think there are literally zero code paths that currently call
> try_to_unmap() that will actually work like that on TDX.  If we run
> out of memory on a TDX host, we can kill the guest completely and
> reclaim all of its memory (which probably also involves killing QEMU
> or whatever other user program is in charge), but that's really our
> only option.

I think our only option for swap is guest co-operation.  We're going to
have to inflate a balloon or something in the guest and have the guest
driver do some type of bounce of the page, where it becomes an
unencrypted page in the guest (so the host can read it without the
physical address keying of the encryption getting in the way) but
actually encrypted with a swap transfer key known only to the guest.  I
assume we can use the page acceptance infrastructure currently being
discussed elsewhere to do swap back in as well ... the host provides
the guest with the encrypted swap page and the guest has to decrypt it
and place it in encrypted guest memory.

That way the swapped memory is securely encrypted, but can be swapped
back in.

James