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Date:   Mon, 4 Apr 2022 17:06:43 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Quentin Perret <qperret@google.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
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Subject: Re: [PATCH v5 00/13] KVM: mm: fd-based approach for supporting KVM
 guest private memory
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On Mon, Apr 04, 2022, Quentin Perret wrote:
> On Friday 01 Apr 2022 at 12:56:50 (-0700), Andy Lutomirski wrote:
> FWIW, there are a couple of reasons why I'd like to have in-place
> conversions:
> 
>  - one goal of pKVM is to migrate some things away from the Arm
>    Trustzone environment (e.g. DRM and the likes) and into protected VMs
>    instead. This will give Linux a fighting chance to defend itself
>    against these things -- they currently have access to _all_ memory.
>    And transitioning pages between Linux and Trustzone (donations and
>    shares) is fast and non-destructive, so we really do not want pKVM to
>    regress by requiring the hypervisor to memcpy things;

Is there actually a _need_ for the conversion to be non-destructive?  E.g. I assume
the "trusted" side of things will need to be reworked to run as a pKVM guest, at
which point reworking its logic to understand that conversions are destructive and
slow-ish doesn't seem too onerous.

>  - it can be very useful for protected VMs to do shared=>private
>    conversions. Think of a VM receiving some data from the host in a
>    shared buffer, and then it wants to operate on that buffer without
>    risking to leak confidential informations in a transient state. In
>    that case the most logical thing to do is to convert the buffer back
>    to private, do whatever needs to be done on that buffer (decrypting a
>    frame, ...), and then share it back with the host to consume it;

If performance is a motivation, why would the guest want to do two conversions
instead of just doing internal memcpy() to/from a private page?  I would be quite
surprised if multiple exits and TLB shootdowns is actually faster, especially at
any kind of scale where zapping stage-2 PTEs will cause lock contention and IPIs.

>  - similar to the previous point, a protected VM might want to
>    temporarily turn a buffer private to avoid ToCToU issues;

Again, bounce buffer the page in the guest.

>  - once we're able to do device assignment to protected VMs, this might
>    allow DMA-ing to a private buffer, and make it shared later w/o
>    bouncing.

Exposing a private buffer to a device doesn't requring in-place conversion.  The
proper way to handle this would be to teach e.g. VFIO to retrieve the PFN from
the backing store.  I don't understand the use case for sharing a DMA'd page at a
later time; with whom would the guest share the page?  E.g. if a NIC has access to
guest private data then there should never be a need to convert/bounce the page.