linux-kernel - Re: [PATCH v3 kvm/queue 05/16] KVM: Maintain ofs_tree for fast memslot lookup by file offset

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Message-ID: <YcS5uStTallwRs0G@google.com>
Date:   Thu, 23 Dec 2021 18:02:33 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chao Peng <chao.p.peng@...ux.intel.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...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>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, john.ji@...el.com, susie.li@...el.com,
        jun.nakajima@...el.com, dave.hansen@...el.com, ak@...ux.intel.com,
        david@...hat.com
Subject: Re: [PATCH v3 kvm/queue 05/16] KVM: Maintain ofs_tree for fast
 memslot lookup by file offset

On Thu, Dec 23, 2021, Chao Peng wrote:
> Similar to hva_tree for hva range, maintain interval tree ofs_tree for
> offset range of a fd-based memslot so the lookup by offset range can be
> faster when memslot count is high.

This won't work.  The hva_tree relies on there being exactly one virtual address
space, whereas with private memory, userspace can map multiple files into the
guest at different gfns, but with overlapping offsets.

I also dislike hijacking __kvm_handle_hva_range() in patch 07.

KVM also needs to disallow mapping the same file+offset into multiple gfns, which
I don't see anywhere in this series.

In other words, there needs to be a 1:1 gfn:file+offset mapping.  Since userspace
likely wants to allocate a single file for guest private memory and map it into
multiple discontiguous slots, e.g. to skip the PCI hole, the best idea off the top
of my head would be to register the notifier on a per-slot basis, not a per-VM
basis.  It would require a 'struct kvm *' in 'struct kvm_memory_slot', but that's
not a huge deal.

That way, KVM's notifier callback already knows the memslot and can compute overlap
between the memslot and the range by reversing the math done by kvm_memfd_get_pfn().
Then, armed with the gfn and slot, invalidation is just a matter of constructing
a struct kvm_gfn_range and invoking kvm_unmap_gfn_range().