Re: [PATCH v3 6/8] KVM: arm64: Add hypervisor overflow stack

From: Fuad Tabba <tabba@google.com>
To: Kalesh Singh <kaleshsingh@google.com>
Cc: will@kernel.org, maz@kernel.org, qperret@google.com,
	surenb@google.com, kernel-team@android.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>,
	Mark Rutland <mark.rutland@arm.com>,
	Mark Brown <broonie@kernel.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Peter Collingbourne <pcc@google.com>,
	"Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>,
	Andrew Walbran <qwandor@google.com>,
	Andrew Scull <ascull@google.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 6/8] KVM: arm64: Add hypervisor overflow stack
Date: Thu, 24 Feb 2022 12:26:33 +0000	[thread overview]
Message-ID: <CA+EHjTwhpjDDnKRU3QrJysgMhYH7eTi2_LrWpxCArLoP-1-BrQ@mail.gmail.com> (raw)
In-Reply-To: <20220224051439.640768-7-kaleshsingh@google.com>

Hi Kalesh,

On Thu, Feb 24, 2022 at 5:21 AM Kalesh Singh <kaleshsingh@google.com> wrote:
>
> Allocate and switch to 16-byte aligned secondary stack on overflow. This
> provides us stack space to better handle overflows; and is used in
> a subsequent patch to dump the hypervisor stacktrace. The overflow stack
> is only allocated if CONFIG_NVHE_EL2_DEBUG is enabled, as hypervisor
> stacktraces is a debug feature dependent on CONFIG_NVHE_EL2_DEBUG.
>
> Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
> ---
>  arch/arm64/kvm/hyp/nvhe/host.S   | 5 +++++
>  arch/arm64/kvm/hyp/nvhe/switch.c | 5 +++++
>  2 files changed, 10 insertions(+)
>
> diff --git a/arch/arm64/kvm/hyp/nvhe/host.S b/arch/arm64/kvm/hyp/nvhe/host.S
> index 749961bfa5ba..367a01e8abed 100644
> --- a/arch/arm64/kvm/hyp/nvhe/host.S
> +++ b/arch/arm64/kvm/hyp/nvhe/host.S
> @@ -179,6 +179,10 @@ SYM_FUNC_END(__host_hvc)
>         b       hyp_panic
>
>  .L__hyp_sp_overflow\@:
> +#ifdef CONFIG_NVHE_EL2_DEBUG
> +       /* Switch to the overflow stack */
> +       adr_this_cpu sp, hyp_overflow_stack + PAGE_SIZE, x0
> +#else
>         /*
>          * Reset SP to the top of the stack, to allow handling the hyp_panic.
>          * This corrupts the stack but is ok, since we won't be attempting
> @@ -186,6 +190,7 @@ SYM_FUNC_END(__host_hvc)
>          */

Nit: Maybe you should update this comment as well, since whether it
corrupts the stack or not depends on what happens above with
CONFIG_NVHE_EL2_DEBUG.

Thanks,
/fuad

>         ldr_this_cpu    x0, kvm_init_params + NVHE_INIT_STACK_HYP_VA, x1
>         mov     sp, x0
> +#endif
>
>         bl      hyp_panic_bad_stack
>         ASM_BUG()
> diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
> index 703a5d3f611b..efc20273a352 100644
> --- a/arch/arm64/kvm/hyp/nvhe/switch.c
> +++ b/arch/arm64/kvm/hyp/nvhe/switch.c
> @@ -34,6 +34,11 @@ DEFINE_PER_CPU(struct kvm_host_data, kvm_host_data);
>  DEFINE_PER_CPU(struct kvm_cpu_context, kvm_hyp_ctxt);
>  DEFINE_PER_CPU(unsigned long, kvm_hyp_vector);
>
> +#ifdef CONFIG_NVHE_EL2_DEBUG
> +DEFINE_PER_CPU(unsigned long [PAGE_SIZE/sizeof(long)], hyp_overflow_stack)
> +       __aligned(16);
> +#endif
> +
>  static void __activate_traps(struct kvm_vcpu *vcpu)
>  {
>         u64 val;
> --
> 2.35.1.473.g83b2b277ed-goog
>