Re: [PATCH] btrace: diagnose "record btrace pt" without libipt

[Pedro Alves <palves at redhat dot com>]

On 11/20/2015 01:59 PM, Metzger, Markus T wrote:

> Thanks for pointing this out.  Let's try to fix it...
> 
> Handling the "E.Btrace already enabled" error in remote.c shouldn't be too hard.
> This would at least allow another "record btrace" after reconnect - and it should
> keep the trace logs.  This takes the same code path as a new enable so the check
> in this patch should suffice.
> 
> A non-PT enabled GDB would try to fall back to BTS, though, so we could either
> implicitly disable PT in the target to allow it or give an error, which again leaves
> branch tracing unusable in this GDB session.  But it would keep the trace logs if
> the user accidentally chose the wrong GDB for reconnecting.
> 
> It would be nice if GDB could detect that record btrace is already enabled and push
> the record-btrace target automatically.  I guess this requires some indication about
> the record status from gdbserver.  Is there some other target that does this automatic
> push on (re-)connect that I could use as reference?

E.g., linux-thread-db.c pushes itself from a new_objfile observer (thread_db_new_objfile),
and  the spu multiarch target pushes itself from an inferior_created observer
(spu_multiarch_inferior_created).

> 
> This still leaves the question how GDB should behave if it doesn't support the tracing
> format that's already enabled in the GDBserver it just connected to.
> 

I guess warn/error when you try to view/consume it, but still let you disable
the tracing?

Thanks,
Pedro Alves