Re: [PATCH v5 17/19] KVM: Terminate memslot walks via used_slots

Peter Xu
2020-02-07T22:25:02Z

On Fri, Feb 07, 2020 at 02:03:25PM -0800, Sean Christopherson wrote:
> On Fri, Feb 07, 2020 at 04:46:23PM -0500, Peter Xu wrote:
> > On Fri, Feb 07, 2020 at 01:10:16PM -0800, Sean Christopherson wrote:
> > > On Fri, Feb 07, 2020 at 03:39:09PM -0500, Peter Xu wrote:
> > > > On Fri, Feb 07, 2020 at 10:33:25AM -0800, Sean Christopherson wrote:
> > > > > On Thu, Feb 06, 2020 at 04:09:44PM -0500, Peter Xu wrote:
> > > > > > On Tue, Jan 21, 2020 at 02:31:55PM -0800, Sean Christopherson wrote:
> > > > > > > @@ -9652,13 +9652,13 @@ int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size)
> > > > > > > if (IS_ERR((void *)hva))
> > > > > > > return PTR_ERR((void *)hva);
> > > > > > > } else {
> > > > > > > - if (!slot->npages)
> > > > > > > + if (!slot || !slot->npages)
> > > > > > > return 0;
> > > > > > >
> > > > > > > - hva = 0;
> > > > > > > + hva = slot->userspace_addr;
> > > > > >
> > > > > > Is this intended?
> > > > >
> > > > > Yes. It's possible to allow VA=0 for userspace mappings. It's extremely
> > > > > uncommon, but possible. Therefore "hva == 0" shouldn't be used to
> > > > > indicate an invalid slot.
> > > >
> > > > Note that this is the deletion path in __x86_set_memory_region() not
> > > > allocation. IIUC userspace_addr won't even be used in follow up code
> > > > path so it shouldn't really matter. Or am I misunderstood somewhere?
> > >
> > > No, but that's precisely why I don't want to zero out @hva, as doing so
> > > implies that '0' indicates an invalid hva, which is wrong.
> > >
> > > What if I change this to
> > >
> > > hva = 0xdeadull << 48;
> > >
> > > and add a blurb in the changelog about stuff hva with a non-canonical value
> > > to indicate it's being destroyed.
> >
> > IMO it's fairly common to have the case where "when A is XXX then
> > parameters B is invalid" happens in C.
>
> I'm not arguing that's not the case. My point is that there's nothing
> special about '0', so why use it? E.g. "hva = 1" would also be ok from a
> functional perspective, but more obviously "wrong".

I think the answer is as simple as the original author thought 0 was
better than an arbitrary number on the stack, which I agree. :-)

--
Peter Xu