Re: [PATCH v10 02/14] KVM: Cache as_id in kvm_memory_slot

Sean Christopherson
2020-07-07T19:57:01Z

On Tue, Jul 07, 2020 at 03:50:09PM -0400, Peter Xu wrote:
> On Mon, Jul 06, 2020 at 11:17:32PM -0700, Sean Christopherson wrote:
> > On Fri, Jul 03, 2020 at 02:41:22PM -0400, Peter Xu wrote:
> > > On Thu, Jul 02, 2020 at 04:08:49PM -0700, Sean Christopherson wrote:
> > > > This technically needs to set as_id in the deleted memslot. I highly doubt
> > > > it will ever matter from a functionality perspective, but it'd be confusing
> > > > to encounter a memslot whose as_id did not match that of its owner.
> > >
> > > Yeah it shouldn't matter because as_id is directly passed in to look up the
> > > pointer of kvm_memslots in kvm_delete_memslot, and memslot->as_id shouldn't be
> > > further referenced.
> > >
> > > I can add a comment above if this can clarify things a bit:
> > >
> > > + u16 as_id; /* cache of as_id; only valid if npages != 0 */
> >
> > Why not just set it?
>
> Because the value is useless even if set? :)

It's useless when things go according to plan, but I can see it being useful
if there's a bug that leads to consumption of a deleted memslot. Maybe not
"useful" so much as "not misleading".

> You mean in kvm_delete_memslot(), am I right?

Yes.

> > It's a single line of code, and there's more than one
> > "shouldn't" in the above.
>
> If you want, I can both set it and add the comment. Thanks,

Why bother with the comment? It'd be wrong in the sense that the as_id is
always valid/accurate, even if npages == 0.