Re: [PATCH] kvm: fix to update memslots properly

Jamie Heilman
2014-12-27T22:53:00Z

Paolo Bonzini wrote:
> This suggests another fix. We can change the insertion to use a ">="
> comparison, as in your first patch. Alone it is not correct, but we
> only need to take some care and avoid breaking the case of deleting a
> memslot.
>
> It's enough to wrap the second loop (that you patched) with
> "if (new->npages)". In the new->npages == 0 case the first loop has
> already set i to the right value, and moving i back would be wrong:
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index f5283438ee05..050974c051b5 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -687,11 +687,23 @@ static void update_memslots(struct kvm_memslots *slots,
> slots->id_to_index[mslots[i].id] = i;
> i++;
> }
> - while (i > 0 &&
> - new->base_gfn > mslots[i - 1].base_gfn) {
> - mslots[i] = mslots[i - 1];
> - slots->id_to_index[mslots[i].id] = i;
> - i--;
> +
> + /*
> + * The ">=" is needed when creating a slot with base_gfn == 0,
> + * so that it moves before all those with base_gfn == npages == 0.
> + *
> + * On the other hand, if new->npages is zero, the above loop has
> + * already left i pointing to the beginning of the empty part of
> + * mslots, and the ">=" would move the hole backwards in this
> + * case---which is wrong. So skip the loop when deleting a slot.
> + */
> + if (new->npages) {
> + while (i > 0 &&
> + new->base_gfn >= mslots[i - 1].base_gfn) {
> + mslots[i] = mslots[i - 1];
> + slots->id_to_index[mslots[i].id] = i;
> + i--;
> + }
> }
>
> mslots[i] = *new;

I gave this a try, and it works just fine for me too.

--
Jamie Heilman http://audible.transient.net/~jamie/