Date:   Tue, 28 Dec 2021 07:50:42 +0800
From:   Yao Yuan <yaoyuan0329os@gmail.com>
To:     Chao Peng <chao.p.peng@linux.intel.com>
Cc:     Sean Christopherson <seanjc@google.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, qemu-devel@nongnu.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
        Hugh Dickins <hughd@google.com>,
        Jeff Layton <jlayton@kernel.org>,
        "J . Bruce Fields" <bfields@fieldses.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        luto@kernel.org, john.ji@intel.com, susie.li@intel.com,
        jun.nakajima@intel.com, dave.hansen@intel.com, ak@linux.intel.com,
        david@redhat.com
Subject: Re: [PATCH v3 kvm/queue 05/16] KVM: Maintain ofs_tree for fast
 memslot lookup by file offset
Message-ID: <20211227235042.rmnwzcqy6ujj75zp@sapienza>
References: <20211223123011.41044-1-chao.p.peng@linux.intel.com>
 <20211223123011.41044-6-chao.p.peng@linux.intel.com>
 <YcS5uStTallwRs0G@google.com>
 <20211224035418.GA43608@chaop.bj.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20211224035418.GA43608@chaop.bj.intel.com>
Precedence: bulk

On Fri, Dec 24, 2021 at 11:54:18AM +0800, Chao Peng wrote:
> 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.

How about pass "context" of fd (e.g. the gfn/hva start point) when register
the invalidation notifier to fd, then in callback kvm can convert the
offset to absolute hva/gfn with such "context", then do memslot invalidation.

>
> Thanks,
> Chao