linux-kernel - Re: [PATCH 2/2] KVM: Scalable memslots implementation

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Message-ID: <9caed824-a3ab-cd23-457e-cb35bb913f91@redhat.com>
Date:   Wed, 3 Feb 2021 14:43:57 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: Scalable memslots implementation

On 03/02/21 00:43, Sean Christopherson wrote:
>> But overall, this solution (and the one with id_to_index moved into the
>> main array, too) is still O(n) per memslot operation as you still need to
>> copy the array to either make space for the new memslot or to remove the
>> hole from the removed memslot.
> 
> Yes, but that problem that can be solved separately from the performance issue
> with hva lookups.

I know the world is not just QEMU, but I'll note that in QEMU all 
memslot operations are roughly O(n) anyway.

> Dumping everything into a single patch makes bisecting nearly worthless, e.g. if
> fast hva lookups breaks a non-x86 architecture, we should able to bisect to
> exactly that, not a massive patch that completely rewrites all of the memslot
> code in one fell swoop.
> 
> Mega patches with multiple logical changes are also extremely difficult to
> review.

Indeed.  This patch does get a +1 for having associated selftests, but 
the diffstat puts it back at the bottom of the pile. :)

Paolo