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

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Date:   Fri, 24 Dec 2021 11:54:18 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.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 at 06:02:33PM +0000, Sean Christopherson wrote:
> 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.

OK, that's the point.

> 
> 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.

This can be checked against file+offset overlapping with existing slots
when register a new one.

> 
> 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().

KVM is easy but the kernel bits would be difficulty, it has to maintain
fd+offset to memslot mapping because one fd can have multiple memslots,
it need decide which memslot needs to be notified.

Thanks,
Chao