Re: [RFC PATCH v3 04/11] contrib/plugins: add plugin showcasing new dicontinuity related API

[Alex Bennée]

Julian Ganz <neither@nut.email> writes:

> We recently introduced new plugin API for registration of discontinuity
> related callbacks. This change introduces a minimal plugin showcasing
> the new API. It simply counts the occurances of interrupts, exceptions
> and host calls per CPU and reports the counts when exitting.
> ---
>  contrib/plugins/meson.build |  3 +-
>  contrib/plugins/traps.c     | 96 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 98 insertions(+), 1 deletion(-)
>  create mode 100644 contrib/plugins/traps.c
>
> diff --git a/contrib/plugins/meson.build b/contrib/plugins/meson.build
> index 63a32c2b4f..9a3015e1c1 100644
> --- a/contrib/plugins/meson.build
> +++ b/contrib/plugins/meson.build
> @@ -1,5 +1,6 @@
>  contrib_plugins = ['bbv', 'cache', 'cflow', 'drcov', 'execlog', 'hotblocks',
> -                   'hotpages', 'howvec', 'hwprofile', 'ips', 'stoptrigger']
> +                   'hotpages', 'howvec', 'hwprofile', 'ips', 'stoptrigger',
> +                   'traps']

I wonder if this is better in tests/tcg/plugins? We need to do something
to ensure it gets covered by CI although we might want to be smarter
about running it together with a test binary that will actually pick up
something.

>  if host_os != 'windows'
>    # lockstep uses socket.h
>    contrib_plugins += 'lockstep'
<snip>
> +QEMU_PLUGIN_EXPORT
> +int qemu_plugin_install(qemu_plugin_id_t id, const qemu_info_t *info,
> +                        int argc, char **argv)
> +{
> +    if (!info->system_emulation) {
> +        fputs("trap plugin can only be used in system emulation mode.\n",
> +              stderr);
> +        return -1;
> +    }
> +
> +    max_vcpus = info->system.max_vcpus;
> +    traps = qemu_plugin_scoreboard_new(sizeof(TrapCounters));
> +    qemu_plugin_register_vcpu_init_cb(id, vcpu_init);
> +    qemu_plugin_vcpu_for_each(id, vcpu_init);

Hmm at first glances this seems redundant - however I guess this is
covering the use case you load the plugin after the system is up and
running.

I wonder if you have unearthed a foot-gun in the API that is easy to
fall into? Maybe we should expand qemu_plugin_register_vcpu_init_cb to
call the call back immediately for existing vcpus?

> +
> +    qemu_plugin_register_vcpu_discon_cb(id, QEMU_PLUGIN_DISCON_TRAPS,
> +                                        vcpu_discon);
> +
> +    qemu_plugin_register_atexit_cb(id, plugin_exit, NULL);
> +
> +    return 0;
> +}

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro