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) {
Hmm, once n_requested_mmu_pages is set it can't be unset. That means this can be
further optimized to skip avoid taking mmu_lock on flags-only changes (and
memslot movement). E.g.
if (!kvm->arch.n_requested_mmu_pages &&
(change == KVM_MR_CREATE || change == KVM_MR_DELETE)) {
}
It's a little risky, but kvm_vm_ioctl_set_nr_mmu_pages() would need to be modified
to allow clearing n_requested_mmu_pages and it already takes slots_lock, so IMO
it's ok to force kvm_vm_ioctl_set_nr_mmu_pages() to recalculate pages if it wants
to allow reverting back to the default.
> + 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);
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);
>
