Re: [PATCH 06/14] KVM: arm64: Tolerate re-creating hyp mappings to set ignored bits

[Fuad Tabba <tabba@google.com>]

Hi Quentin,

On Tue, Jul 20, 2021 at 11:30 AM 'Quentin Perret' via kernel-team
<kernel-team@android.com> wrote:
>
> Hi Fuad,
>
> On Tuesday 20 Jul 2021 at 11:17:03 (+0100), Fuad Tabba wrote:
> > Hi Quentin,
> >
> >
> > On Mon, Jul 19, 2021 at 11:47 AM Quentin Perret <qperret@google.com> wrote:
> > >
> > > The current hypervisor stage-1 mapping code doesn't allow changing an
> > > existing valid mapping. Relax this condition by allowing changes that
> > > only target ignored bits, as that will soon be needed to annotate shared
> > > pages.
> > >
> > > Signed-off-by: Quentin Perret <qperret@google.com>
> > > ---
> > >  arch/arm64/kvm/hyp/pgtable.c | 18 ++++++++++++++++--
> > >  1 file changed, 16 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > > index a0ac8c2bc174..34cf67997a82 100644
> > > --- a/arch/arm64/kvm/hyp/pgtable.c
> > > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > > @@ -362,6 +362,17 @@ static int hyp_set_prot_attr(enum kvm_pgtable_prot prot, kvm_pte_t *ptep)
> > >         return 0;
> > >  }
> > >
> > > +static bool hyp_pte_needs_update(kvm_pte_t old, kvm_pte_t new)
> > > +{
> > > +       if (old == new)
> > > +               return false;
> > > +
> > > +       if (!kvm_pte_valid(old))
> > > +               return true;
> > > +
> > > +       return !WARN_ON((old ^ new) & ~KVM_PTE_LEAF_ATTR_IGNORED);
> >
> > Wouldn't this return false if both ignored and non-ignored bits were
> > different, or is that not possible (judging by the WARN_ON)?
>
> Correct, but that is intentional, see below ;)
>
> > If it is, then it would need an update, wouldn't it?
>
> Maybe, but if you look at what the existing code does, we do skip the
> update if the old mapping is valid and not equal to new. So I kept the
> behaviour as close as possible to this -- if you change any bits outside
> of SW bits you get a WARN and we skip the update, as we already do
> today. But if you touch only SW bits and nothing else, then I let the
> update go through.
>
> That said, I don't think warning and then proceeding to update would be
> terribly wrong, it's just that a change of behaviour felt a bit
> unnecessary for this particular patch.

Thanks for the clarification. It makes sense to preserve the existing
behavior, but I was wondering if a comment would be good, describing
what merits a "needs update"?

Cheers,
/fuad

> Thanks,
> Quentin
>
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