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Date:   Wed, 20 Oct 2021 19:01:51 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Igor Mammedov <imammedo@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        James Morse <james.morse@arm.com>,
        Julien Thierry <julien.thierry.kdev@gmail.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 01/13] KVM: x86: Cache total page count to avoid
 traversing the memslot array
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References: <cover.1632171478.git.maciej.szmigiero@oracle.com>
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On Wed, Oct 20, 2021, Maciej S. Szmigiero wrote:
> On 20.10.2021 00:24, Sean Christopherson wrote:
> > E.g. the whole thing can be
> > 
> > 	if (!kvm->arch.n_requested_mmu_pages &&
> > 	    (change == KVM_MR_CREATE || change == KVM_MR_DELETE)) {
> > 		unsigned long nr_mmu_pages;
> > 
> > 		if (change == KVM_MR_CREATE) {
> > 			kvm->arch.n_memslots_pages += new->npages;
> > 		} else {
> > 			WARN_ON(kvm->arch.n_memslots_pages < old->npages);
> > 			kvm->arch.n_memslots_pages -= old->npages;
> > 		}
> > 
> > 		nr_mmu_pages = (unsigned long)kvm->arch.n_memslots_pages;
> > 		nr_mmu_pages *= (KVM_PERMILLE_MMU_PAGES / 1000);
> 
> The above line will set nr_mmu_pages to zero since KVM_PERMILLE_MMU_PAGES
> is 20, so when integer-divided by 1000 will result in a multiplication
> coefficient of zero.

Ugh, math.  And thus do_div() to avoid the whole 64-bit divide issue on 32-bit KVM.
Bummer.