On Wed, Jan 07, 2026 at 03:40:22PM +0000, Marc Zyngier wrote:

On Tue, 02 Dec 2025 09:36:22 +0000,
Vincent Donnefort [off-list ref] wrote:

quoted

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 <redacted>

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.

I could add to the event header the VM pid related to the currently loaded vCPU
(if any). I can access it easily with host_kvm->userspace_pid. WDYS?

Thanks,

	M.

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