From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Chen, Tiejun" Subject: Re: [PATCH] kvm: fix to update memslots properly Date: Mon, 29 Dec 2014 09:06:09 +0800 Message-ID: <54A0A901.40209@intel.com> References: <1419569710-8127-1-git-send-email-tiejun.chen@intel.com> <549F1989.5050305@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit To: Paolo Bonzini , KVM list , Andy Lutomirski , jamie@audible.transient.net, Igor Mammedov Return-path: Received: from mga14.intel.com ([192.55.52.115]:50723 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751496AbaL2BGM (ORCPT ); Sun, 28 Dec 2014 20:06:12 -0500 In-Reply-To: <549F1989.5050305@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 2014/12/28 4:41, 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. Yeah, but its really a little ugly to see some slots, base_gfn:npages:falgs = 0:0:(!0), to resort again when debug something inside of update_memslots(). > >> @@ -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 Yeah. > 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; > This looks better. Tiejun