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    SubjectRe: [PATCH v5.5 20/30] KVM: x86: Use nr_memslot_pages to avoid traversing the memslots array
    Date
    On 04.11.2021 01:25, Sean Christopherson wrote:
    > From: Maciej S. Szmigiero <maciej.szmigiero@oracle.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.
    >
    > Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
    > [sean: use common KVM field and rework changelog accordingly]
    > Signed-off-by: Sean Christopherson <seanjc@google.com>
    > ---
    > arch/x86/include/asm/kvm_host.h | 1 -
    > arch/x86/kvm/mmu/mmu.c | 24 ------------------------
    > arch/x86/kvm/x86.c | 11 ++++++++---
    > 3 files changed, 8 insertions(+), 28 deletions(-)
    >
    (..)
    > --- a/arch/x86/kvm/x86.c
    > +++ b/arch/x86/kvm/x86.c
    > @@ -11837,9 +11837,14 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
    > enum kvm_mr_change change)
    > {
    > if (!kvm->arch.n_requested_mmu_pages &&
    > - (change == KVM_MR_CREATE || change == KVM_MR_DELETE))
    > - kvm_mmu_change_mmu_pages(kvm,
    > - kvm_mmu_calculate_default_mmu_pages(kvm));
    > + (change == KVM_MR_CREATE || change == KVM_MR_DELETE)) {
    > + unsigned long nr_mmu_pages;
    > +
    > + nr_mmu_pages = kvm->nr_memslot_pages * KVM_PERMILLE_MMU_PAGES;

    Unfortunately, even if kvm->nr_memslot_pages is capped at ULONG_MAX then
    this value multiplied by 20 can still overflow an unsigned long variable.

    Thanks,
    Maciej

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