Re: [Qemu-devel] [PATCH 1/2] trace: Add "cpu_init" event

[Lluís Vilanova]

Stefan Hajnoczi writes:

> On Wed, Sep 14, 2016 at 06:01:17PM +0200, Lluís Vilanova wrote:
>> Stefan Hajnoczi writes:
>> 
>> > On Tue, Sep 06, 2016 at 04:25:53PM +0200, Lluís Vilanova wrote:
>> >> +## vCPU
>> >> +
>> >> +# Create a new virtual (guest) CPU
>> >> +#
>> >> +# Targets: all
>> >> +guest_cpu_init(void *cpu) "cpu=%p"
>> 
>> > This isn't a vcpu trace event.  Please add keep it with the other
>> > non-vcpu trace events:
>> 
>> > # cpus.c
>> > guest_cpu_init(void *cpu) "cpu=%p"
>> 
>> It actually is, but as the commit message says, declaring it as such prevents
>> the event to be emitted.

> If it cannot be toggled on a per-vcpu basis then claiming it's a vcpu
> event doesn't make sense.

>> The culprit of this problem is that new vCPUs start with an empty per-vCPU 
>> trace
>> event set. Should we make vCPUs "inherit" the state from the global state?
>> (i.e., if any vcpu event is set on any vCPU, set it on the new one). The next
>> question would then be, should this inheritance only apply until tracing is
>> fully initialized of for the whole duration of QEMU?

> I think the underlying issue is that trace_init_vcpu_events() assumes
> there is a single instant where vcpus all exist and need to be
> initialized.

> A model that supports vcpu hotplug is really needed.  In that model the
> global dstate should be the "global" state that determines whether vcpus
> are initialized with the event enabled or disabled.

Ok, so then you're going for what I was saying. "Inheriting" the state of every
new vCPU from the global dynamic state. If *any* vCPU has an event enabled, all
new vCPUs will too.

As I said, next question is whether inheritance should apply only until full
initialization, or the whole time. I thing the latter is less confusing.


Cheers,
  Lluis