Re: Unexport of kvm_x86_ops vs tracer modules

Paolo Bonzini
2022-04-08T16:24:17Z

On 4/8/22 17:36, Mathieu Desnoyers wrote:
> LTTng is an out of tree kernel module, which currently relies on the export.
> Indeed, arch/x86/kvm/x86.c exports a set of tracepoints to kernel modules, e.g.:
>
> EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_entry)
>
> But any probe implementation hooking on that tracepoint would need kvm_x86_ops
> to translate the struct kvm_vcpu * into meaningful tracing data.
>
> I could work-around this on my side in ugly ways, but I would like to discuss
> how kernel module tracers are expected to implement kvm events probes without
> the kvm_x86_ops symbol ?

The conversion is done in the TP_fast_assign snippets, which are part of
kvm.ko and therefore do not need the export. As I understand it, the
issue is that LTTng cannot use the TP_fast_assign snippets, because they
are embedded in the trace_event_raw_event_* symbols?

We cannot do the extraction before calling trace_kvm_exit, because it's
expensive.

Paolo