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

From: Andy Lutomirski
Date: Wed Sep 01 2021 - 13:09:08 EST

On Wed, Sep 1, 2021, at 9:18 AM, James Bottomley wrote:
> 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.

I asked David, and he said the PSP offers a swapping mechanism for SEV-ES. I haven’t read the details, but they should all be public.