Re: [PATCH v4 08/18] KVM: arm64: Add PROTECTED_NVHE_STACKTRACE Kconfig

[Kalesh Singh]

On Tue, Jul 19, 2022 at 3:35 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Mon, 18 Jul 2022 18:03:30 +0100,
> Kalesh Singh <kaleshsingh@xxxxxxxxxx> wrote:
> >
> > On Sun, Jul 17, 2022 at 11:56 PM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > >
> > > [- Drew and android-mm, as both addresses bounce]
> > >
> > > On Fri, 15 Jul 2022 07:10:17 +0100,
> > > Kalesh Singh <kaleshsingh@xxxxxxxxxx> wrote:
> > > >
> > > > This can be used to disable stacktrace for the protected KVM
> > > > nVHE hypervisor, in order to save on the associated memory usage.
> > > >
> > > > This option is disabled by default, since protected KVM is not widely
> > > > used on platforms other than Android currently.
> > > >
> > > > Signed-off-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
> > > > ---
> > > >  arch/arm64/kvm/Kconfig | 15 +++++++++++++++
> > > >  1 file changed, 15 insertions(+)
> > > >
> > > > diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> > > > index 8a5fbbf084df..1edab6f8a3b8 100644
> > > > --- a/arch/arm64/kvm/Kconfig
> > > > +++ b/arch/arm64/kvm/Kconfig
> > > > @@ -46,6 +46,21 @@ menuconfig KVM
> > > >
> > > >         If unsure, say N.
> > > >
> > > > +config PROTECTED_NVHE_STACKTRACE
> > > > +     bool "Protected KVM hypervisor stacktraces"
> > > > +     depends on KVM
> > > > +     default n
> > > > +     help
> > > > +       Say Y here to enable pKVM hypervisor stacktraces on hyp_panic()
> > > > +
> > > > +       If you are not using protected nVHE (pKVM), say N.
> > > > +
> > > > +       If using protected nVHE mode, but cannot afford the associated
> > > > +       memory cost (less than 0.75 page per CPU) of pKVM stacktraces,
> > > > +       say N.
> > > > +
> > > > +       If unsure, say N.
> > > > +
> > >
> > > Can we make this depend on NVHE_EL2_DEBUG instead? I'd like to keep
> > > the disclosing of EL2 information in protected mode a strict debug
> > > feature.
> >
> > Hi Marc,
> >
> > An earlier version was similar to what you propose. The unwinding
> > depended on NVHE_EL2_DEBUG and all unwinding was done from EL1 with
> > host stage 2 being disabled. The reason the design was changed is
> > because Android expressed the need for pKVM hyp stacktraces in
> > production environments. [1]
>
> I think that's an Android-specific requirement that doesn't apply to
> upstream. If Android wants to enable this in production (and
> potentially leak details of the hypervisor address space), that's
> Android's business, and they can carry a patch for that.  Upstream
> shouldn't have to cater for such a thing.

Hi Marc,

For android it's important to be able to debug issues from the field.
But I agree no need to subject upstream to the same requirements. I'll
guard this with the NVHE_EL2_DEBUG config in the next version.

Thanks,
Kalesh

>
> Thanks,
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.