linux-kernel - Re: [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

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Message-ID: <ZUFGRyQEuWj4RJS0@google.com>
Date:   Tue, 31 Oct 2023 11:24:07 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Xiaoyao Li <xiaoyao.li@...el.com>,
        Xu Yilun <yilun.xu@...el.com>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Anish Moorthy <amoorthy@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        Mickaël Salaün <mic@...ikod.net>,
        Vlastimil Babka <vbabka@...e.cz>,
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        Maciej Szmigiero <mail@...iej.szmigiero.name>,
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        Quentin Perret <qperret@...gle.com>,
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        Isaku Yamahata <isaku.yamahata@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for
 guest-specific backing memory

On 2023-10-27 11:21 AM, Sean Christopherson wrote:
> Introduce an ioctl(), KVM_CREATE_GUEST_MEMFD, to allow creating file-based
> memory that is tied to a specific KVM virtual machine and whose primary
> purpose is to serve guest memory.
> 
> A guest-first memory subsystem allows for optimizations and enhancements
> that are kludgy or outright infeasible to implement/support in a generic
> memory subsystem.  With guest_memfd, guest protections and mapping sizes
> are fully decoupled from host userspace mappings.   E.g. KVM currently
> doesn't support mapping memory as writable in the guest without it also
> being writable in host userspace, as KVM's ABI uses VMA protections to
> define the allow guest protection.  Userspace can fudge this by
> establishing two mappings, a writable mapping for the guest and readable
> one for itself, but that’s suboptimal on multiple fronts.
> 
> Similarly, KVM currently requires the guest mapping size to be a strict
> subset of the host userspace mapping size, e.g. KVM doesn’t support
> creating a 1GiB guest mapping unless userspace also has a 1GiB guest
> mapping.  Decoupling the mappings sizes would allow userspace to precisely
> map only what is needed without impacting guest performance, e.g. to
> harden against unintentional accesses to guest memory.
> 
> Decoupling guest and userspace mappings may also allow for a cleaner
> alternative to high-granularity mappings for HugeTLB, which has reached a
> bit of an impasse and is unlikely to ever be merged.
> 
> A guest-first memory subsystem also provides clearer line of sight to
> things like a dedicated memory pool (for slice-of-hardware VMs) and
> elimination of "struct page" (for offload setups where userspace _never_
> needs to mmap() guest memory).

All of these use-cases involve using guest_memfd for shared pages, but
this entire series sets up KVM to only use guest_memfd for private
pages.

For example, the per-page attributes are a property of a KVM VM, not the
underlying guest_memfd. So that implies we will need separate
guest_memfds for private and shared pages. But a given memslot can have
a mix of private and shared pages. So that implies a memslot will need
to support 2 guest_memfds? But the UAPI only allows 1 and uses the HVA
for shared mappings.

My initial reaction after reading through this series is that the
per-page private/shared should be a property of the guest_memfd, not the
VM. Maybe it would even be cleaner in the long-run to make all memory
attributes a property of the guest_memfd. That way we can scope the
support to only guest_memfds and not have to worry about making per-page
attributes work with "legacy" HVA-based memslots.

Maybe can you sketch out how you see this proposal being extensible to
using guest_memfd for shared mappings?