Re: [PATCH 3/6] perf tools: Add guest_code support

[Adrian Hunter]

On 17/05/22 06:13, Namhyung Kim wrote:
> Hi Adrian,
> 
> On Fri, May 13, 2022 at 2:03 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>>
>> A common case for KVM test programs is that the guest object code can be
>> found in the hypervisor process (i.e. the test program running on the
>> host). To support that, copy the host thread's maps to the guest thread's
>> maps. Note, we do not discover the guest until we encounter a guest event,
>> which works well because it is not until then that we know that the host
>> thread's maps have been set up.
>>
>> Typically the main function for the guest object code is called
>> "guest_code", hence the name chosen for this feature.
> 
> Ok, so that's just a convention and there's no hard-coded
> support for the "guest_code" function in this code, right?

That is correct.

> 
>>
>> This is primarily aimed at supporting Intel PT, or similar, where trace
>> data can be recorded for a guest. Refer to the final patch in this series
>> "perf intel-pt: Add guest_code support" for an example.
>>
>> Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
>> ---
>>  tools/perf/util/event.c       |  7 +++-
>>  tools/perf/util/machine.c     | 70 +++++++++++++++++++++++++++++++++++
>>  tools/perf/util/machine.h     |  2 +
>>  tools/perf/util/session.c     |  7 ++++
>>  tools/perf/util/symbol_conf.h |  3 +-
>>  5 files changed, 86 insertions(+), 3 deletions(-)
>>
>> diff --git a/tools/perf/util/event.c b/tools/perf/util/event.c
>> index 6439c888ae38..0476bb3a4188 100644
>> --- a/tools/perf/util/event.c
>> +++ b/tools/perf/util/event.c
>> @@ -683,9 +683,12 @@ static bool check_address_range(struct intlist *addr_list, int addr_range,
>>  int machine__resolve(struct machine *machine, struct addr_location *al,
>>                      struct perf_sample *sample)
>>  {
>> -       struct thread *thread = machine__findnew_thread(machine, sample->pid,
>> -                                                       sample->tid);
>> +       struct thread *thread;
>>
>> +       if (symbol_conf.guest_code && !machine__is_host(machine))
>> +               thread = machine__findnew_guest_code(machine, sample->pid);
>> +       else
>> +               thread = machine__findnew_thread(machine, sample->pid, sample->tid);
>>         if (thread == NULL)
>>                 return -1;
>>
>> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
>> index e67b5a7670f3..ae2e1fb422e2 100644
>> --- a/tools/perf/util/machine.c
>> +++ b/tools/perf/util/machine.c
>> @@ -392,6 +392,76 @@ struct machine *machines__find_guest(struct machines *machines, pid_t pid)
>>         return machine;
>>  }
>>
>> +/*
>> + * A common case for KVM test programs is that the guest object code can be
>> + * found in the hypervisor process (i.e. the test program running on the host).
>> + * To support that, copy the host thread's maps to the guest thread's maps.
>> + * Note, we do not discover the guest until we encounter a guest event,
>> + * which works well because it is not until then that we know that the host
>> + * thread's maps have been set up.
>> + */
>> +static struct thread *findnew_guest_code(struct machine *machine,
> 
> But this function returns a thread and IIUC that's the task which
> does the host to guest transition.  Then why not calling it just
> findnew__hypervisor() ?

The thread returned is in the guest machine.  While the code comes
from the hypervisor, it is in the guest VM when it runs.

>From Intel PT point of view, this function allows finding the guest
object code by setting up the guest thread and its maps.

I will try to improve on the explanation in V2.

> 
> Thanks,
> Namhyung
> 
> 
>> +                                        struct machine *host_machine,
>> +                                        pid_t pid)
>> +{
>> +       struct thread *host_thread;
>> +       struct thread *thread;
>> +       int err;
>> +
>> +       if (!machine)
>> +               return NULL;
>> +
>> +       thread = machine__findnew_thread(machine, -1, pid);
>> +       if (!thread)
>> +               return NULL;
>> +
>> +       /* Assume maps are set up if there are any */
>> +       if (thread->maps->nr_maps)
>> +               return thread;
>> +
>> +       host_thread = machine__find_thread(host_machine, -1, pid);
>> +       if (!host_thread)
>> +               goto out_err;
>> +
>> +       thread__set_guest_comm(thread, pid);
>> +
>> +       /*
>> +        * Guest code can be found in hypervisor process at the same address
>> +        * so copy host maps.
>> +        */
>> +       err = maps__clone(thread, host_thread->maps);
>> +       thread__put(host_thread);
>> +       if (err)
>> +               goto out_err;
>> +
>> +       return thread;
>> +
>> +out_err:
>> +       thread__zput(thread);
>> +       return NULL;
>> +}
>> +