On 19/08/2018 02:17, Wei Yang wrote:
> This is a trivial optimization of the first loop in update_memslots().
>
> Since used_slots records the number of valid slots, it could be used as the
> boundary of the loop instead of looping the whole array and check npages.
>
> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
> ---
> virt/kvm/kvm_main.c | 6 ++----
> 1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 5c1911790f25..fac3225eff35 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -806,15 +806,13 @@ static void update_memslots(struct kvm_memslots *slots,
> {
> int id = new->id;
> int i = slots->id_to_index[id];
> + int used_slots = slots->used_slots;
> struct kvm_memory_slot *mslots = slots->memslots;
>
> WARN_ON(mslots[i].id != id);
> slots->used_slots += (char)change;
>
> - while (i < KVM_MEM_SLOTS_NUM - 1 &&
> - new->base_gfn <= mslots[i + 1].base_gfn) {
> - if (!mslots[i + 1].npages)
> - break;
> + while (i < used_slots - 1 && new->base_gfn <= mslots[i + 1].base_gfn) {
> mslots[i] = mslots[i + 1];
> slots->id_to_index[mslots[i].id] = i;
> i++;
>
The loop is removing an element from the array, so the correct limit for
the index would be the *old* length of the array, not the new one.
Alternatively... just keep the current code. :) It is already loading
mslots[i+1] to get the base_gfn, so the load is not expensive. This is
also not really a fast path.
Paolo
