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

Peter Xu
2020-02-07T21:46:31Z

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. OK feel free to keep any of
these as you prefer (how many times I spoke this only for today? :) as
long as the maintainers are fine with it. And for sure an extra
comment would always be nice.

Thanks,

--
Peter Xu