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

[Sean Christopherson]

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.