Re: [RFC PATCH v3 01/11] plugins: add types for callbacks related to certain discontinuities

[Alex Bennée]

Pierrick Bouvier <pierrick.bouvier@linaro.org> writes:

> Hi Julian,
>
> thanks for the update!
> Comments below.
>
> On 12/2/24 11:26, Julian Ganz wrote:
>> The plugin API allows registration of callbacks for a variety of VCPU
>> related events, such as VCPU reset, idle and resume. However, traps of
>> any kind, i.e. interrupts or exceptions, were previously not covered.
>> These kinds of events are arguably quite significant and usually go hand
>> in hand with a PC discontinuity. On most platforms, the discontinuity
>> also includes a transition from some "mode" to another. Thus, plugins
>> for the analysis of (virtualized) embedded systems may benefit from or
>> even require the possiblity to perform work on the occurance of an
>> interrupt or exception.
>> This change introduces the concept of such a discontinuity event in
>> the
>> form of an enumeration. Currently only traps are covered. Specifically
>> we (loosely) define interrupts, exceptions and host calls across all
>> platforms. In addition, this change introduces a type to use for
>> callback functions related to such events. Since possible modes and the
>> enumeration of interupts and exceptions vary greatly between different
>> architectures, the callback type only receives the VCPU id, the type of
>> event as well as the old and new PC.
>> ---
>>   include/qemu/plugin.h      |  1 +
>>   include/qemu/qemu-plugin.h | 43 ++++++++++++++++++++++++++++++++++++++
>>   2 files changed, 44 insertions(+)
>> diff --git a/include/qemu/plugin.h b/include/qemu/plugin.h
>> index 9726a9ebf3..27a176b631 100644
>> --- a/include/qemu/plugin.h
>> +++ b/include/qemu/plugin.h
>> @@ -59,6 +59,7 @@ union qemu_plugin_cb_sig {
>>       qemu_plugin_udata_cb_t           udata;
>>       qemu_plugin_vcpu_simple_cb_t     vcpu_simple;
>>       qemu_plugin_vcpu_udata_cb_t      vcpu_udata;
>> +    qemu_plugin_vcpu_discon_cb_t     vcpu_discon;
>>       qemu_plugin_vcpu_tb_trans_cb_t   vcpu_tb_trans;
>>       qemu_plugin_vcpu_mem_cb_t        vcpu_mem;
>>       qemu_plugin_vcpu_syscall_cb_t    vcpu_syscall;
>> diff --git a/include/qemu/qemu-plugin.h b/include/qemu/qemu-plugin.h
>> index 0fba36ae02..9c67374b7e 100644
>> --- a/include/qemu/qemu-plugin.h
>> +++ b/include/qemu/qemu-plugin.h
>> @@ -154,6 +154,49 @@ typedef void 
>> (*qemu_plugin_vcpu_simple_cb_t)(qemu_plugin_id_t id,
>>   typedef void (*qemu_plugin_vcpu_udata_cb_t)(unsigned int vcpu_index,
>>                                               void *userdata);
>>   +
>> +/**
>> + * enum qemu_plugin_discon_type - type of a (potential) PC discontinuity
>> + *
>> + * @QEMU_PLUGIN_DISCON_INTERRUPT: an interrupt, defined across all 
>> architectures
>> + *                                as an asynchronous event, usually 
>> originating
>> + *                                from outside the CPU
>> + * @QEMU_PLUGIN_DISCON_EXCEPTION: an exception, defined across all 
>> architectures
>> + *                                as a synchronous event in response to a
>> + *                                specific instruction being executed
>> + * @QEMU_PLUGIN_DISCON_HOSTCALL: a host call, functionally a special kind of
>> + *                               exception that is not handled by code run 
>> by
>> + *                               the vCPU but machinery outside the vCPU
>> + * @QEMU_PLUGIN_DISCON_ALL: all types of disconinuity events currently 
>> covered
>> + */
>> +enum qemu_plugin_discon_type {
>> +    QEMU_PLUGIN_DISCON_INTERRUPT = 1,
>> +    QEMU_PLUGIN_DISCON_EXCEPTION = 2,
>> +    QEMU_PLUGIN_DISCON_HOSTCALL = 4,
>> +    QEMU_PLUGIN_DISCON_ALL = 7
>> +};
>
> Matter of style, but would be better to use:
>
> enum qemu_plugin_discon_type {
>      QEMU_PLUGIN_DISCON_INTERRUPT = 1 << 0,
>      QEMU_PLUGIN_DISCON_EXCEPTION = 1 << 1,
>      QEMU_PLUGIN_DISCON_HOSTCALL = 1 << 2,
>      QEMU_PLUGIN_DISCON_ALL = -1
> };
>
<snip>

Is this really a bit field though? If you will only report type of
discontinuity at a time a simple 0 based enum with
QEMU_PLUGIN_DISCON_MAX would be simpler.

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro