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Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private
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To:     jejb@linux.ibm.com, Andy Lutomirski <luto@kernel.org>,
        Sean Christopherson <seanjc@google.com>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <a259e10d-39c9-c4a5-0ab4-f42a1b9bfaee@redhat.com>
Date:   Wed, 1 Sep 2021 18:22:06 +0200
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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, ...).

-- 
Thanks,

David / dhildenb