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

[Kalesh Singh <kaleshsingh@google.com>]

On Sun, Jul 17, 2022 at 11:56 PM Marc Zyngier <maz@kernel.org> wrote:
>
> [- Drew and android-mm, as both addresses bounce]
>
> On Fri, 15 Jul 2022 07:10:17 +0100,
> Kalesh Singh <kaleshsingh@google.com> 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@google.com>
> > ---
> >  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]

[1] https://lore.kernel.org/all/CAC_TJveNRaDFcQGo9-eqKa3=1DnuVDs4U+ye795VcJ1ozVkMyg@mail.gmail.com/

--Kalesh

>
> >  config NVHE_EL2_DEBUG
> >       bool "Debug mode for non-VHE EL2 object"
> >       depends on KVM
>
> Thanks,
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.