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

Marcelo Tosatti
2015-03-09T20:55:15Z

On Sat, Dec 27, 2014 at 09:41:45PM +0100, Paolo Bonzini wrote:
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index f528343..6e52f3f 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -672,6 +672,7 @@ static void update_memslots(struct kvm_memslots *slots,
> > WARN_ON(mslots[i].id != id);
> > if (!new->npages) {
> > new->base_gfn = 0;
> > + new->flags = 0;
> > if (mslots[i].npages)
> > slots->used_slots--;
> > } else {
>
> This should not be necessary. The part of the mslots array that has
> base_gfn == npages == 0 is entirely unused, and such a slot can never
> be returned by search_memslots because this:
>
> if (gfn >= memslots[slot].base_gfn &&
> gfn < memslots[slot].base_gfn + memslots[slot].npages)
>
> can never be true.
>
> > @@ -688,7 +689,9 @@ static void update_memslots(struct kvm_memslots *slots,
> > i++;
> > }
> > while (i > 0 &&
> > - new->base_gfn > mslots[i - 1].base_gfn) {
> > + ((new->base_gfn > mslots[i - 1].base_gfn) ||
> > + (!new->base_gfn &&
> > + !mslots[i - 1].base_gfn && !mslots[i - 1].npages))) {
> > mslots[i] = mslots[i - 1];
> > slots->id_to_index[mslots[i].id] = i;
> > i--;
> >
>
> You should have explained _why_ this fixes the bug, and what invariant
> is not being respected, something like this:
>
> kvm: fix sorting of memslots with base_gfn == 0
>
> Before commit 0e60b0799fed (kvm: change memslot sorting rule from size
> to GFN, 2014-12-01), the memslots' sorting key was npages, meaning
> that a valid memslot couldn't have its sorting key equal to zero.
> On the other hand, a valid memslot can have base_gfn == 0, and invalid
> memslots are identified by base_gfn == npages == 0.
>
> Because of this, commit 0e60b0799fed broke the invariant that invalid
> memslots are at the end of the mslots array. When a memslot with
> base_gfn == 0 was created, any invalid memslot before it were left
> in place.
>
> 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;
>
> Paolo

Paolo,

Can you include a proper changelog for this patch?