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 10:08 -0700, Andy Lutomirski wrote:
> 
> 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.

Well it does, but it's not useful: we can't use the PSP for bulk
encryption, it's too slow.  That's why we're having to fuss about fast
migration in the first place.  In theory the two PSPs can co-operate to
migrate a guest but only if you have about a year to wait for it to
happen.

James