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References: <20220715061027.1612149-1-kaleshsingh@google.com> <20220715061027.1612149-10-kaleshsingh@google.com>
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From:   Fuad Tabba <tabba@google.com>
Date:   Mon, 18 Jul 2022 11:00:03 +0100
Message-ID: <CA+EHjTyY5ieRROhC9S-WzUybcp3j8egJLGpCHV_2vSt8ieUjXg@mail.gmail.com>
Subject: Re: [PATCH v4 09/18] KVM: arm64: Allocate shared pKVM hyp stacktrace buffers
To:     Kalesh Singh <kaleshsingh@google.com>
Cc:     maz@kernel.org, mark.rutland@arm.com, broonie@kernel.org,
        madvenka@linux.microsoft.com, will@kernel.org, qperret@google.com,
        james.morse@arm.com, alexandru.elisei@arm.com,
        suzuki.poulose@arm.com, catalin.marinas@arm.com,
        andreyknvl@gmail.com, russell.king@oracle.com,
        vincenzo.frascino@arm.com, mhiramat@kernel.org, ast@kernel.org,
        wangkefeng.wang@huawei.com, elver@google.com, keirf@google.com,
        yuzenghui@huawei.com, ardb@kernel.org, oupton@google.com,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-kernel@vger.kernel.org, kernel-team@android.com
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Hi Kalesh,

On Fri, Jul 15, 2022 at 7:11 AM Kalesh Singh <kaleshsingh@google.com> wrote:
>
> In protected nVHE mode the host cannot directly access
> hypervisor memory, so we will dump the hypervisor stacktrace
> to a shared buffer with the host.
>
> The minimum size do the buffer required, assuming the min frame
> size of [x29, x30] (2 * sizeof(long)), is half the combined size of
> the hypervisor and overflow stacks plus an additional entry to
> delimit the end of the stacktrace.
>
> The stacktrace buffers are used later in the seried to dump the
> nVHE hypervisor stacktrace when using protected-mode.
>
> Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
> ---
>  arch/arm64/include/asm/memory.h      | 7 +++++++
>  arch/arm64/kvm/hyp/nvhe/stacktrace.c | 4 ++++
>  2 files changed, 11 insertions(+)
>
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index 0af70d9abede..28a4893d4b84 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -113,6 +113,13 @@
>
>  #define OVERFLOW_STACK_SIZE    SZ_4K
>
> +/*
> + * With the minimum frame size of [x29, x30], exactly half the combined
> + * sizes of the hyp and overflow stacks is needed to save the unwinded
> + * stacktrace; plus an additional entry to delimit the end.
> + */
> +#define NVHE_STACKTRACE_SIZE   ((OVERFLOW_STACK_SIZE + PAGE_SIZE) / 2 + sizeof(long))

I do find this computation to be a bit confusing, especially with the
addition of the entry to delimit the end. Especially when considering
that in patch 12, where you add pkvm_save_backtrace_entry(), you need
to compensate for it again.

Not sure what the best way is, having two definitions, or something
like that, with one for the size and one for the delimiter.

Thanks,
/fuad

> +
>  /*
>   * Alignment of kernel segments (e.g. .text, .data).
>   *
> diff --git a/arch/arm64/kvm/hyp/nvhe/stacktrace.c b/arch/arm64/kvm/hyp/nvhe/stacktrace.c
> index a3d5b34e1249..69e65b457f1c 100644
> --- a/arch/arm64/kvm/hyp/nvhe/stacktrace.c
> +++ b/arch/arm64/kvm/hyp/nvhe/stacktrace.c
> @@ -9,3 +9,7 @@
>
>  DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)], overflow_stack)
>         __aligned(16);
> +
> +#ifdef CONFIG_PROTECTED_NVHE_STACKTRACE
> +DEFINE_PER_CPU(unsigned long [NVHE_STACKTRACE_SIZE/sizeof(long)], pkvm_stacktrace);
> +#endif /* CONFIG_PROTECTED_NVHE_STACKTRACE */
> --
> 2.37.0.170.g444d1eabd0-goog
>