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    SubjectRe: [PATCH v4 10/18] KVM: arm64: Stub implementation of pKVM HYP stack unwinder
    On Fri, 15 Jul 2022 07:10:19 +0100,
    Kalesh Singh <kaleshsingh@google.com> wrote:
    >
    > Add some stub implementations of protected nVHE stack unwinder, for
    > building. These are implemented later in this series.
    >
    > Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
    > ---
    > arch/arm64/include/asm/stacktrace/nvhe.h | 57 ++++++++++++++++++++++++
    > arch/arm64/kvm/hyp/nvhe/stacktrace.c | 3 +-
    > 2 files changed, 58 insertions(+), 2 deletions(-)
    > create mode 100644 arch/arm64/include/asm/stacktrace/nvhe.h
    >
    > diff --git a/arch/arm64/include/asm/stacktrace/nvhe.h b/arch/arm64/include/asm/stacktrace/nvhe.h
    > new file mode 100644
    > index 000000000000..1eac4e57f2ae
    > --- /dev/null
    > +++ b/arch/arm64/include/asm/stacktrace/nvhe.h
    > @@ -0,0 +1,57 @@
    > +/* SPDX-License-Identifier: GPL-2.0-only */
    > +/*
    > + * KVM nVHE hypervisor stack tracing support.
    > + *
    > + * The unwinder implementation depends on the nVHE mode:
    > + *
    > + * 1) pKVM (protected nVHE) mode - the host cannot directly access
    > + * the HYP memory. The stack is unwinded in EL2 and dumped to a shared
    > + * buffer where the host can read and print the stacktrace.
    > + *
    > + * Copyright (C) 2022 Google LLC
    > + */
    > +#ifndef __ASM_STACKTRACE_NVHE_H
    > +#define __ASM_STACKTRACE_NVHE_H
    > +
    > +#include <asm/stacktrace/common.h>
    > +
    > +static inline bool on_accessible_stack(const struct task_struct *tsk,
    > + unsigned long sp, unsigned long size,
    > + struct stack_info *info)
    > +{
    > + return false;
    > +}
    > +
    > +/*
    > + * Protected nVHE HYP stack unwinder
    > + */
    > +#ifdef __KVM_NVHE_HYPERVISOR__
    > +
    > +#ifdef CONFIG_PROTECTED_NVHE_STACKTRACE
    > +static inline bool on_overflow_stack(unsigned long sp, unsigned long size,
    > + struct stack_info *info)
    > +{
    > + return false;
    > +}
    > +
    > +static int notrace unwind_next(struct unwind_state *state)
    > +{
    > + return 0;
    > +}
    > +NOKPROBE_SYMBOL(unwind_next);

    I find this rather dodgy. It means that every compilation unit that
    (indirectly) drags this include file may end-up with an 'unwind_next'
    function. At best this will be eliminated at compilation time, but it
    may also generate a warning.

    Why can't this me made an 'inline' function? At the very least, it
    should have a __maybe_unused attribute.

    > +#else /* !CONFIG_PROTECTED_NVHE_STACKTRACE */
    > +static inline bool on_overflow_stack(unsigned long sp, unsigned long size,
    > + struct stack_info *info)
    > +{
    > + return false;
    > +}
    > +
    > +static int notrace unwind_next(struct unwind_state *state)
    > +{
    > + return 0;
    > +}
    > +NOKPROBE_SYMBOL(unwind_next);

    Same thing here.

    Thanks,

    M.

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

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