linux-kernel - Re: [PATCH v4 11/18] KVM: arm64: Stub implementation of non-protected nVHE HYP stack unwinder

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Message-ID: <54b1c642097af7d290174072bd6e856c@kernel.org>
Date:   Mon, 18 Jul 2022 17:57:25 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Kalesh Singh <kaleshsingh@...gle.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Mark Brown <broonie@...nel.org>,
        "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>,
        Will Deacon <will@...nel.org>,
        Quentin Perret <qperret@...gle.com>,
        Fuad Tabba <tabba@...gle.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        andreyknvl@...il.com, russell.king@...cle.com,
        vincenzo.frascino@....com, Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Marco Elver <elver@...gle.com>, Keir Fraser <keirf@...gle.com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Oliver Upton <oupton@...gle.com>,
        "moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" 
        <linux-arm-kernel@...ts.infradead.org>,
        kvmarm <kvmarm@...ts.cs.columbia.edu>,
        LKML <linux-kernel@...r.kernel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>
Subject: Re: [PATCH v4 11/18] KVM: arm64: Stub implementation of non-protected
 nVHE HYP stack unwinder

On 2022-07-18 17:51, Kalesh Singh wrote:
> On Mon, Jul 18, 2022 at 12:31 AM Marc Zyngier <maz@...nel.org> wrote:
>> 
>> On Fri, 15 Jul 2022 07:10:20 +0100,
>> Kalesh Singh <kaleshsingh@...gle.com> wrote:
>> >
>> > Add stub implementations of non-protected nVHE stack unwinder, for
>> > building. These are implemented later in this series.
>> >
>> > Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>
>> > ---
>> >  arch/arm64/include/asm/stacktrace/nvhe.h | 22 ++++++++++++++++++++++
>> >  1 file changed, 22 insertions(+)
>> >
>> > diff --git a/arch/arm64/include/asm/stacktrace/nvhe.h b/arch/arm64/include/asm/stacktrace/nvhe.h
>> > index 1eac4e57f2ae..36cf7858ddd8 100644
>> > --- a/arch/arm64/include/asm/stacktrace/nvhe.h
>> > +++ b/arch/arm64/include/asm/stacktrace/nvhe.h
>> > @@ -8,6 +8,12 @@
>> >   *      the HYP memory. The stack is unwinded in EL2 and dumped to a shared
>> >   *      buffer where the host can read and print the stacktrace.
>> >   *
>> > + *   2) Non-protected nVHE mode - the host can directly access the
>> > + *      HYP stack pages and unwind the HYP stack in EL1. This saves having
>> > + *      to allocate shared buffers for the host to read the unwinded
>> > + *      stacktrace.
>> > + *
>> > + *
>> >   * Copyright (C) 2022 Google LLC
>> >   */
>> >  #ifndef __ASM_STACKTRACE_NVHE_H
>> > @@ -53,5 +59,21 @@ static int notrace unwind_next(struct unwind_state *state)
>> >  NOKPROBE_SYMBOL(unwind_next);
>> >  #endif       /* CONFIG_PROTECTED_NVHE_STACKTRACE */
>> >
>> > +/*
>> > + * Non-protected nVHE HYP stack unwinder
>> > + */
>> > +#else        /* !__KVM_NVHE_HYPERVISOR__ */
>> 
>> I don't get this path. This either represents the VHE hypervisor or
>> the kernel proper. Which one is it?
> 
> Hi Marc. This is run from kernel proper context. And it's the
> unwinding for conventional nVHE (non-protected). The unwinding is done
> from the host kernel in EL1.

This really deserves a comment here, as the one you currently
have is a bit misleading (and you probably want to move it
below the #else).

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...