Re: How to use bts recording?

[Simon Sobisch <simonsobisch@gnu.org>]

Am 26.01.2022 um 18:11 schrieb Metzger, Markus T:
> Hello Simon,
> 
>>> Are you maybe using virtualization?
>>
>> Rechecked with the server team: yes, the old RHEL kernel runs on Redhat
>> Virtualization Manager; the newer one on VMware vSphere.
> 
> Here's the problem.  AFAIK neither BTS nor PT are virtualized.  This is simply
> missing enabling in hypervisors.  They do properly indicate this by omitting
> the respective cpuid enumeration.  This results in an EOPNOTSUPP when
> GDB tries to enable it, which is printed by GDB.

I see - but something is still not ideal.
At least on this machine GDB 11.1 said:

(gdb) record btrace bts
Could not enable branch tracing for Thread 0x7ffff7fe4740 (LWP 2756): 
**Failed to start recording: Operation not supported**

While the Debian GNU gdb (Debian 10.1-1.7) 10.1.90.20210103-git reports

Could not enable branch tracing for Thread 0x7ffff79833c0 (LWP 334): 
**BTS support has been disabled for the target cpu**

That's a clear message, I like that.

Is this a special Debian patch or is because of the target cpu being AMD?
Is there a path that should be taken when the OS reports EOPNOTSUPP?

A message similar to the one above would be useful.

> I'm afraid that's a fairly common problem, these days, but I don't see how
> GDB could check that it is running in a guest OS and that the hypervisor did
> not virtualize PT or BTS.

As you've said: use of virtualized environments is quite common, so it 
may be reasonable to adjust the docs to at least hint at that issue.

I suggest changing

- Hardware-supported instruction recording, supported on Intel processors
+ Hardware-supported instruction recording, supported on Intel 
processors if not running in a virtual machine.

... or something like that.


BTW: rechecked newer RHEL:
GNU gdb (GDB) Red Hat Enterprise Linux 8.2-12.el8 also has the link to 
libipt and so also says
Failed to open /sys/bus/event_source/devices/intel_pt/type

[note: that message possibly could hint at "Intel PT not available"]

Also: not sure if libipt is the one that is checking that directory, but 
possibly GDB could do this _before_ printing
 > Intel Processor Trace support was disabled at compile time.

... because this led me to believe "just reconfigure and possible 
install libipt before and everything will be fine" - but if I had done 
this the only result would have been another error message.