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

[David Hildenbrand <david@redhat.com>]

On 01.09.21 18:31, James Bottomley wrote:
> 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:

Yes, I heard of that. "Interesting" design.

> 
> https://patchew.org/EDK2/20210818212048.162626-1-tobin@linux.ibm.com/
> 
> 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?

Right, but that's than no longer ballooning, unless I am missing 
something important. You'd ask the guest to export/import, and you can 
trust it. But do we want to call something like that out of random 
kernel context when swapping/writeback, ...? Hard to tell. Feels like it 
won't win in a beauty contest.

-- 
Thanks,

David / dhildenb