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

[James Bottomley]

On Wed, 2021-09-01 at 18:22 +0200, David Hildenbrand wrote:
> On 01.09.21 18:18, 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.
> 
> Ballooning is indeed *the* mechanism to avoid swapping in the
> hypervisor  and much rather let the guest swap. Shame it requires
> trusting a guest, which we, in general, can't. Not to mention other
> issues we already do have with ballooning (latency, broken auto-
> ballooning, over-inflating, ...).


Well not necessarily, but it depends how clever we want to get.  If you
look over on the OVMF/edk2 list, there's a proposal to do guest
migration via a mirror VM that invokes a co-routine embedded in the
OVMF binary:

https://patchew.org/EDK2/20210818212048.162626-1-tobin@xxxxxxxxxxxxx/

This gives us a page encryption mechanism that's provided by the host
but accepted via the guest using attestation, meaning we have a
mutually trusted piece of code that can use to extract encrypted pages.
It does seem it could be enhanced to do swapping for us as well if
that's a road we want to go down?

James