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

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Date:   Fri, 3 Nov 2023 16:17:23 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Fuad Tabba <tabba@...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>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Anish Moorthy <amoorthy@...gle.com>,
        David Matlack <dmatlack@...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>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        David Hildenbrand <david@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Wang <wei.w.wang@...el.com>,
        Liam Merwick <liam.merwick@...cle.com>,
        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 Thu, Nov 02, 2023, Fuad Tabba wrote:
> On Wed, Nov 1, 2023 at 9:55 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > E.g. a misbehaving userspace could prematurely delete a memslot.  And the more
> > fun example is intrahost migration, where the plan is to allow pointing multiple
> > guest_memfd files at a single guest_memfd inode:
> > https://lore.kernel.org/all/cover.1691446946.git.ackerleytng@google.com
> >
> > There was a lot of discussion for this, but it's scattered all over the place.
> > The TL;DR is is that the inode will represent physical memory, and a file will
> > represent a given "struct kvm" instance's view of that memory.  And so the memory
> > isn't reclaimed until the inode is truncated/punched.
> >
> > I _think_ this reflects the most recent plan from the guest_memfd side:
> > https://lore.kernel.org/all/1233d749211c08d51f9ca5d427938d47f008af1f.1689893403.git.isaku.yamahata@intel.com

Doh, sitting in my TODO folder...

https://lore.kernel.org/all/20231016115028.996656-1-michael.roth@amd.com

> Thanks for pointing that out. I think this might be the way to go.
> I'll have a closer look at this and see how to get it to work with
> pKVM.