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

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