linux-kernel - Re: [PATCH v5 00/13] KVM: mm: fd-based approach for supporting KVM guest private memory

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Date:   Mon, 4 Apr 2022 17:06:43 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Quentin Perret <qperret@...gle.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Steven Price <steven.price@....com>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        kvm list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>, qemu-devel@...gnu.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH v5 00/13] KVM: mm: fd-based approach for supporting KVM
 guest private memory

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.