linux-kernel - Re: [PATCH v8 2/8] KVM: Extend the memslot to support fd-based private memory

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Message-ID: <Y0A+rogB6TRDtbyE@google.com>
Date:   Fri, 7 Oct 2022 14:58:54 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     Chao Peng <chao.p.peng@...ux.intel.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-doc@...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>,
        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>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "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>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>, wei.w.wang@...el.com
Subject: Re: [PATCH v8 2/8] KVM: Extend the memslot to support fd-based
 private memory

On Fri, Oct 07, 2022, Jarkko Sakkinen wrote:
> On Thu, Oct 06, 2022 at 03:34:58PM +0000, Sean Christopherson wrote:
> > On Thu, Oct 06, 2022, Jarkko Sakkinen wrote:
> > > On Thu, Oct 06, 2022 at 05:58:03PM +0300, Jarkko Sakkinen wrote:
> > > > On Thu, Sep 15, 2022 at 10:29:07PM +0800, Chao Peng wrote:
> > > > > This new extension, indicated by the new flag KVM_MEM_PRIVATE, adds two
> > > > > additional KVM memslot fields private_fd/private_offset to allow
> > > > > userspace to specify that guest private memory provided from the
> > > > > private_fd and guest_phys_addr mapped at the private_offset of the
> > > > > private_fd, spanning a range of memory_size.
> > > > > 
> > > > > The extended memslot can still have the userspace_addr(hva). When use, a
> > > > > single memslot can maintain both private memory through private
> > > > > fd(private_fd/private_offset) and shared memory through
> > > > > hva(userspace_addr). Whether the private or shared part is visible to
> > > > > guest is maintained by other KVM code.
> > > > 
> > > > What is anyway the appeal of private_offset field, instead of having just
> > > > 1:1 association between regions and files, i.e. one memfd per region?
> > 
> > Modifying memslots is slow, both in KVM and in QEMU (not sure about Google's VMM).
> > E.g. if a vCPU converts a single page, it will be forced to wait until all other
> > vCPUs drop SRCU, which can have severe latency spikes, e.g. if KVM is faulting in
> > memory.  KVM's memslot updates also hold a mutex for the entire duration of the
> > update, i.e. conversions on different vCPUs would be fully serialized, exacerbating
> > the SRCU problem.
> > 
> > KVM also has historical baggage where it "needs" to zap _all_ SPTEs when any
> > memslot is deleted.
> > 
> > Taking both a private_fd and a shared userspace address allows userspace to convert
> > between private and shared without having to manipulate memslots.
> 
> Right, this was really good explanation, thank you.
> 
> Still wondering could this possibly work (or not):
> 
> 1. Union userspace_addr and private_fd.

No, because userspace needs to be able to provide both userspace_addr (shared
memory) and private_fd (private memory) for a single memslot.

> 2. Instead of introducing private_offset, use guest_phys_addr as the
>    offset.

No, because that would force userspace to use a single private_fd for all of guest
memory since it effectively means private_offset=0.  And userspace couldn't skip
over holes in guest memory, i.e. the size of the memfd would need to follow the
max guest gpa.  In other words, dropping private_offset could work, but it'd be
quite kludgy and not worth saving 8 bytes.