Re: [PATCH v9 29/30] KVM: arm64: Add selftest event support to nVHE/pKVM hyp

[Marc Zyngier <maz@kernel.org>]

On Tue, 02 Dec 2025 09:36:22 +0000,
Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> Add a selftest event that can be triggered from a `write_event` tracefs
> file. This intends to be used by trace remote selftests.
> 
> Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> 
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index 37c015b54ff6..e2de7ad64938 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -96,6 +96,7 @@ enum __kvm_host_smccc_func {
>  	__KVM_HOST_SMCCC_FUNC___tracing_update_clock,
>  	__KVM_HOST_SMCCC_FUNC___tracing_reset,
>  	__KVM_HOST_SMCCC_FUNC___tracing_enable_event,
> +	__KVM_HOST_SMCCC_FUNC___tracing_write_event,
>  };
>  
>  #define DECLARE_KVM_VHE_SYM(sym)	extern char sym[]
> diff --git a/arch/arm64/include/asm/kvm_hypevents.h b/arch/arm64/include/asm/kvm_hypevents.h
> index 268b3cd7a1b2..c276fd28e0bf 100644
> --- a/arch/arm64/include/asm/kvm_hypevents.h
> +++ b/arch/arm64/include/asm/kvm_hypevents.h
> @@ -42,4 +42,15 @@ HYP_EVENT(hyp_exit,
>  	),
>  	HE_PRINTK("reason=%s", __hyp_enter_exit_reason_str(__entry->reason))
>  );
> +
> +HYP_EVENT(selftest,
> +	HE_PROTO(u64 id),
> +	HE_STRUCT(
> +		he_field(u64, id)
> +	),
> +	HE_ASSIGN(
> +		__entry->id = id;
> +	),
> +	RE_PRINTK("id=%llu", __entry->id)

Not strictly related to this patch, but I find that the trace itself
lacks context. For example:

[001]	  323.847422: hyp_enter reason=hvc
[001]	  323.847423: hyp_exit reason=eret_host
[001]	  323.847688: hyp_enter reason=hvc
[001]	  323.847688: hyp_exit reason=eret_host
[001]	  323.847706: hyp_enter reason=hvc
[001]	  323.847707: hyp_exit reason=eret_host
[001]	  323.847722: hyp_enter reason=hvc
[001]	  323.847723: hyp_exit reason=eret_host

That's all fine as long as I'm dealing with a single guest, or even
with a single vcpu. Trying to trace multiple guests, or even multiple
vcpus makes the whole thing completely unusable, because I have no
idea what I'm looking at.

To make this useful, having some context provided by the host really
is required.

Thanks,

	M.

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