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From:   Kalesh Singh <kaleshsingh@google.com>
Date:   Mon, 18 Jul 2022 10:03:30 -0700
Message-ID: <CAC_TJvcyb6xQhvuoSHpC6+LmeSAqbDk9Fnb=x-bB7UxxMek3EA@mail.gmail.com>
Subject: Re: [PATCH v4 08/18] KVM: arm64: Add PROTECTED_NVHE_STACKTRACE Kconfig
To:     Marc Zyngier <maz@kernel.org>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Mark Brown <broonie@kernel.org>,
        "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>,
        Will Deacon <will@kernel.org>,
        Quentin Perret <qperret@google.com>,
        Fuad Tabba <tabba@google.com>,
        James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        andreyknvl@gmail.com, russell.king@oracle.com,
        vincenzo.frascino@arm.com, Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Marco Elver <elver@google.com>, Keir Fraser <keirf@google.com>,
        Zenghui Yu <yuzenghui@huawei.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Oliver Upton <oupton@google.com>,
        "moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" 
        <linux-arm-kernel@lists.infradead.org>,
        kvmarm <kvmarm@lists.cs.columbia.edu>,
        LKML <linux-kernel@vger.kernel.org>,
        "Cc: Android Kernel" <kernel-team@android.com>
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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.