RE: How to use bts recording?

Hello Simon,

>PT has the downside of GDB having to be configured for that - which I
>guess isn't common in distro versions - and somehow a manual build n the
>4.x kernel where libipt was available also did not pick it up (will try
>to force it with a GDB 11.2 build soon, thanks for pointing out the
>configure flags; I've looked in the tarball in the ./configure file but
>that was in gdb/configure).

Let's work on that, then.  Fedora configures GDB with PT support and I thought RHEL would too.  Not sure at which version they started.  The Debian GDB maintainer promised to enable it but I have not checked, since.


>>> How can I check for BTS support?
>>
>> Hardware support is enumerated by cpuid and published by the kernel in
>/proc/cpuinfo under flags (look for 'bts').  Same for PT (look for 'intel_pt').
>
>That may be the reason that it wasn't picked up; those aren't in there:
>
>flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
>mca cmov pat pse36 clflush mmx fxsr sse sse2 ss syscall nx pdpe1gb
>rdtscp lm constant_tsc arch_perfmon nopl xtopology tsc_reliable
>nonstop_tsc cpuid pni pclmulqdq ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic
>movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand hypervisor
>lahf_lm abm 3dnowprefetch cpuid_fault invpcid_single pti ssbd ibrs ibpb
>stibp fsgsbase tsc_adjust bmi1 avx2 smep bmi2 invpcid avx512f avx512dq
>rdseed adx smap clflushopt clwb avx512cd avx512bw avx512vl xsaveopt
>xsavec xsaves arat pku ospke flush_l1d arch_capabilities
>bugs            : cpu_meltdown spectre_v1 spectre_v2 spec_store_bypass l1tf
>
>flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
>mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx rdtscp lm
>constant_tsc rep_good nopl eagerfpu pni pclmulqdq ssse3 fma cx16 pcid
>sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c
>rdrand hypervisor lahf_lm abm 3dnowprefetch arat fsgsbase bmi1 hle avx2
>smep bmi2 erms invpcid rtm rdseed adx smap xsaveopt

Are you maybe using virtualization?


>> For PT, kernel support could be checked by looking at
>/sys/bus/event_source/devices/intel_pt/type, which gives the perf event type to
>be used for PT.  BTS does not need that as it is using a fixed type.
>
>There's no /sys/bus/event_source/devices/intel_pt/ either.

OK, so your system definitely does not support PT.  This also explains this error...

>event syntax error:
>    'intel_pt//u'
>     \___ Cannot find PMU `intel_pt'. Missing kernel support?
>
>
>I guess in this case there is no use in trying to build with intel-pt,
>is it?

Yes.  You should be able to build GDB with PT support but it wouldn't
work on that system.


>Thanks for trying to get this working, ideally we have some hints the
>next time someone looks out for this and can add a better error message
>for bts to GDB.

If this turns out to be something that GDB can check with reasonable effort,
diagnosing this would certainly help.  Like we do for perf_event_paranoid.

Regards,
Markus.
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