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.
