linux-kernel - Re: [PATCH v6 18/29] KVM: x86: Use nr_memslot_pages to avoid traversing the memslots array

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Message-ID: <YabcNaCb88s/CTop@google.com>
Date:   Wed, 1 Dec 2021 02:21:41 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        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>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Atish Patra <atish.patra@....com>,
        Ben Gardon <bgardon@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 18/29] KVM: x86: Use nr_memslot_pages to avoid
 traversing the memslots array

On Tue, Nov 30, 2021, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
> 
> There is no point in recalculating from scratch the total number of pages
> in all memslots each time a memslot is created or deleted.  Use KVM's
> cached nr_memslot_pages to compute the default max number of MMU pages.
> 
> Note that even with nr_memslot_pages capped at ULONG_MAX we can't safely
> multiply it by KVM_PERMILLE_MMU_PAGES (20) since this operation can
> possibly overflow an unsigned long variable.
> 
> Write this "* 20 / 1000" operation as "/ 50" instead to avoid such
> overflow.
> 
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
> [sean: use common KVM field and rework changelog accordingly]
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

My SoB can definitely be dropped for this one, just consider it review feedback
that happened to have an SoB attached.

Reviewed-by: Sean Christopherson <seanjc@...gle.com>