linux-kernel - Re: [PATCH v5 01/13] KVM: x86: Cache total page count to avoid traversing the memslot array

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Message-ID: <YXBnn6ZaXbaqKvOo@google.com>
Date:   Wed, 20 Oct 2021 19:01:51 +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>,
        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 v5 01/13] KVM: x86: Cache total page count to avoid
 traversing the memslot array

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.