Re: [PATCH] configure: check for perf_event.h version

[Pedro Alves <palves at redhat dot com>]

On 08/06/2015 03:14 PM, Metzger, Markus T wrote:
>> -----Original Message-----
>> From: Pedro Alves [mailto:palves@redhat.com]
>> Sent: Thursday, August 6, 2015 3:49 PM
>> To: Metzger, Markus T
>> Cc: gdb-patches@sourceware.org
>> Subject: Re: [PATCH] configure: check for perf_event.h version
>>
>> On 08/06/2015 02:06 PM, Markus Metzger wrote:
>>> Intel(R) Processor Trace support requires a recent linux/perf_event.h
>> header.
>>>
>>> When GDB is built on an older system, Intel(R) Processor Trace will not be
>>> available and there is no indication in the configure and build log as to
>>> what went wrong.
>>>
>>> Check for a compatible linux/perf_event.h at configure-time.
>>
>>
>>> diff --git a/gdb/configure.ac b/gdb/configure.ac
>>> index 905c27b..d867e85 100644
>>> --- a/gdb/configure.ac
>>> +++ b/gdb/configure.ac
>>> @@ -1252,6 +1252,20 @@ if test "${with_intel_pt}" = no; then
>>>    AC_MSG_WARN([Intel(R) Processor Trace support disabled; some
>> features may be unavailable.])
>>>    HAVE_LIBIPT=no
>>>  else
>>> +  AC_PREPROC_IFELSE(AC_LANG_SOURCE([[
>>> +#include <linux/perf_event.h>
>>> +#ifdef PERF_ATTR_SIZE_VER5
>>> +# error
>>> +#endif
>>
>> Can you explain what kind of symbol PERF_ATTR_SIZE_VER5 is?
>> From the patch, I understand that that is something that is _not_
>> defined in the perf versions that are supposedly supported?
>> (otherwise, I'd expect an #ifndef instead.)
> 
> It's a macro. 

Was there ever a  PERF_ATTR_SIZE_VER4 symbol?   What if
PERF_ATTR_SIZE_VER6 is ever created?  Do we want to have to
check for it explicitly then?

> I took the double-negation approach from a similar
> check for python_has_threads.  

In the python case, we've already checked earlier that python.h
is available.

In your case, it seems that if <linux/perf_event.h> isn't
available, you end up with perf_event=yes?  Is that what we want?

> Maybe this wasn't such a good idea.

Note also that the Python code has an explicit comment
to help people not get confused with the negation:

  # Note that the test is reversed so that python_has_threads=yes on
  # unexpected failures.

If this goes with the double-negation approach, a similar
comment would be very nice to have.

Thanks,
Pedro Alves