Re: [PATCH v4 09/18] KVM: arm64: Allocate shared pKVM hyp stacktrace buffers

From: Marc Zyngier <maz@kernel.org>
To: Kalesh Singh <kaleshsingh@google.com>
Cc: mark.rutland@arm.com, broonie@kernel.org,
	madvenka@linux.microsoft.com, will@kernel.org,
	qperret@google.com, tabba@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
Subject: Re: [PATCH v4 09/18] KVM: arm64: Allocate shared pKVM hyp stacktrace buffers
Date: Mon, 18 Jul 2022 08:13:00 +0100	[thread overview]
Message-ID: <87bktm51xf.wl-maz@kernel.org> (raw)
In-Reply-To: <20220715061027.1612149-10-kaleshsingh@google.com>

On Fri, 15 Jul 2022 07:10:18 +0100,
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

s/do/for/ ?

> 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.

Let me see if I understand this: the maximum stack size is the
combination of the HYP and overflow stacks, and the smallest possible
stack frame is 128bit (only FP+LR). The buffer thus needs to provide
one 64bit entry per stack frame that fits in the combined stack, plus
one entry as an end marker.

So the resulting size is half of the combined stack size, plus a
single 64bit word. Is this correct?

> 
> 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))
> +
>  /*
>   * 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 */

OK, so the allocation exists even if KVM is not running in protected
mode. I guess this is OK for now, but definitely reinforces my request
that this is only there when compiled for debug mode.

Thanks,

	M.

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

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