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

[Maciej S. Szmigiero]

On 20.10.2021 00:24, Sean Christopherson wrote:
> On Mon, Sep 20, 2021, Maciej S. Szmigiero wrote:
> > From: "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx>
> >
> > There is no point in recalculating from scratch the total number of pages
> > in all memslots each time a memslot is created or deleted.
> >
> > Just cache the value and update it accordingly on each such operation so
> > the code doesn't need to traverse the whole memslot array each time.
> >
> > Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
> > ---
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 28ef14155726..65fdf27b9423 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -11609,9 +11609,23 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
> >   				const struct kvm_memory_slot *new,
> >   				enum kvm_mr_change change)
> >   {
> > -	if (!kvm->arch.n_requested_mmu_pages)
> > -		kvm_mmu_change_mmu_pages(kvm,
> > -				kvm_mmu_calculate_default_mmu_pages(kvm));
> > +	if (change == KVM_MR_CREATE)
> > +		kvm->arch.n_memslots_pages += new->npages;
> > +	else if (change == KVM_MR_DELETE) {
> > +		WARN_ON(kvm->arch.n_memslots_pages < old->npages);
> > +		kvm->arch.n_memslots_pages -= old->npages;
> > +	}
> > +
> > +	if (!kvm->arch.n_requested_mmu_pages) {
(..)
> > +		u64 memslots_pages;
> > +		unsigned long nr_mmu_pages;
> > +
> > +		memslots_pages = kvm->arch.n_memslots_pages * KVM_PERMILLE_MMU_PAGES;
> > +		do_div(memslots_pages, 1000);
> > +		nr_mmu_pages = max_t(typeof(nr_mmu_pages),
> > +				     memslots_pages, KVM_MIN_ALLOC_MMU_PAGES);
>
> "memslots_pages" is a bit of a misnomer.  Any objection to avoiding naming problems
> by explicitly casting to an "unsigned long" and simply operating on nr_mmu_pages?
>
> 		nr_mmu_pages = (unsigned long)kvm->arch.n_memslots_pages;
> 		nr_mmu_pages *= (KVM_PERMILLE_MMU_PAGES / 1000);
> 		nr_mmu_pages = max(nr_mmu_pages, KVM_MIN_ALLOC_MMU_PAGES);
> 		kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
>
> 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.

> 		nr_mmu_pages = max(nr_mmu_pages, KVM_MIN_ALLOC_MMU_PAGES);
> 		kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
> 	}
>
> > +		kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
> > +	}
> >   
> >   	kvm_mmu_slot_apply_flags(kvm, old, new, change);
> >   

Thanks,
Maciej