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

[Pierrick Bouvier]

On 12/2/24 11:26, Julian Ganz wrote:
> 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']
>   if host_os != 'windows'
>     # lockstep uses socket.h
>     contrib_plugins += 'lockstep'
> diff --git a/contrib/plugins/traps.c b/contrib/plugins/traps.c
> new file mode 100644
> index 0000000000..ecd4beac5f
> --- /dev/null
> +++ b/contrib/plugins/traps.c
> @@ -0,0 +1,96 @@
> +/*
> + * Copyright (C) 2024, Julian Ganz <neither@nut.email>
> + *
> + * Traps - count traps
> + *
> + * License: GNU GPL, version 2 or later.
> + *   See the COPYING file in the top-level directory.
> + */
> +
> +#include <stdio.h>
> +
> +#include <qemu-plugin.h>
> +
> +QEMU_PLUGIN_EXPORT int qemu_plugin_version = QEMU_PLUGIN_VERSION;
> +
> +typedef struct {
> +    uint64_t interrupts;
> +    uint64_t exceptions;
> +    uint64_t hostcalls;
> +    bool active;

The active field can be removed, as you can query 
qemu_plugin_num_vcpus() to know (dynamically) how many vcpus were created.

> +} TrapCounters;
> +
> +static struct qemu_plugin_scoreboard *traps;
> +static size_t max_vcpus;
> +
> +static void vcpu_init(qemu_plugin_id_t id, unsigned int vcpu_index)
> +{
> +    TrapCounters* rec = qemu_plugin_scoreboard_find(traps, vcpu_index);
> +    rec->active = true;
> +}

Once active is removed, this function can be removed too.

> +
> +static void vcpu_discon(qemu_plugin_id_t id, unsigned int vcpu_index,
> +                        enum qemu_plugin_discon_type type, uint64_t from_pc,
> +                        uint64_t to_pc)
> +{
> +    TrapCounters* rec = qemu_plugin_scoreboard_find(traps, vcpu_index);
> +    switch (type) {
> +    case QEMU_PLUGIN_DISCON_INTERRUPT:
> +        rec->interrupts++;
> +        break;
> +    case QEMU_PLUGIN_DISCON_EXCEPTION:
> +        rec->exceptions++;
> +        break;
> +    case QEMU_PLUGIN_DISCON_HOSTCALL:
> +        rec->hostcalls++;
> +        break;
> +    default:
> +        /* unreachable */

You can use g_assert_not_reached() ensure it's unreachable. We removed 
assert(0) from the codebase and replaced those with it.

> +        break;
> +    }
> +}
> +
> +static void plugin_exit(qemu_plugin_id_t id, void *p)
> +{
> +    g_autoptr(GString) report;
> +    report = g_string_new("VCPU, interrupts, exceptions, hostcalls\n");
> +    int vcpu;
> +
> +    for (vcpu = 0; vcpu < max_vcpus; vcpu++) {

vcpu < qemu_plugin_num_vcpus()

> +        TrapCounters *rec = qemu_plugin_scoreboard_find(traps, vcpu);
> +        if (rec->active) {
> +            g_string_append_printf(report,
> +                                   "% 4d, % 10"PRId64", % 10"PRId64", % 10"
> +                                   PRId64"\n",
> +                                   vcpu,
> +                                   rec->interrupts, rec->exceptions,
> +                                   rec->hostcalls);
> +        }
> +    }
> +
> +    qemu_plugin_outs(report->str);
> +    qemu_plugin_scoreboard_free(traps);
> +}
> +
> +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);
> +
> +    qemu_plugin_register_vcpu_discon_cb(id, QEMU_PLUGIN_DISCON_TRAPS,
> +                                        vcpu_discon);
> +

The change from QEMU_PLUGIN_DISCON_TRAPS to QEMU_PLUGIN_DISCON_ALL 
should be included in this patch, instead of next one.

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