Re: [Qemu-devel] [PATCH v4 00/20] instrument: Add basic event instrumentation

[Emilio G. Cota]

On Wed, Sep 06, 2017 at 20:22:41 +0300, Lluís Vilanova wrote:
> This series adds an API to add instrumentation events.
> 
> It also provides additional APIs for:
> * Controlling tracing events

hmm didn't Stefan say that tracing should be decoupled from this?

> * Peek/poke guest memory
> 
> There's still missing APIs for (can be added in later series?):
> * Provide something like tracing's per-vCPU trace states (i.e., so that each
>   vCPU can have different instrumentation code). It's still not clear to me if
>   we should extend the per-vCPU bitmap with instrumentation events, or 
> otherwise
>   somehow reuse the bits in tracing events (since they're currently limited).

As I said in the description of my alternative implementation [*], I don't see
much value in having per-vCPU events, as most instrumenters just care about
the guest application/system. I can only think of cases where the instrumenter
is only interested in a guest process (in system-mode), but that'd be ugly
anyway (would need to change both QEMU and the guest kernel to track the pid).

If the need ever arises, we could add __vcpu(cpu_index) registration functions.

[*] https://lists.gnu.org/archive/html/qemu-devel/2017-09/msg01446.html

                E.