RE: [PATCH v2 17/17] infrun: scheduler-locking reverse

["Metzger, Markus T" <markus dot t dot metzger at intel dot com>]

> -----Original Message-----
> From: Eli Zaretskii [mailto:eliz@gnu.org]
> Sent: Friday, September 11, 2015 11:02 AM
> To: Pedro Alves
> Cc: Metzger, Markus T; gdb-patches@sourceware.org
> Subject: Re: [PATCH v2 17/17] infrun: scheduler-locking reverse
> 
> > Date: Fri, 11 Sep 2015 09:55:41 +0100
> > From: Pedro Alves <palves@redhat.com>
> > CC: gdb-patches@sourceware.org

The second paragraph of [1] says "When this [record] target is in use, if the
execution log includes the record for the next instruction, gdb will debug in
replay mode.".

Following this definition of "replay mode", I agree with Eli's first suggestion
to call the new scheduler-locking mode "replay".

I'm avoiding the term "replay" since esp. record btrace does not replay in the
sense of "re-execute the program".  All it does is move the PC around.  Record
full does a bit more but it is still not re-executing the program.


> > I'll admit that when reviewing this, I also noticed that the setting
> > applies both when stepping backwards, and when replaying forward,
> > and wondered whether that would be confusing.
> >
> > Do we call reverse execution "replay" too?  The docs are a bit confusing
> > on this, sometimes it looks like we do, sometimes not.  E.g., [1]
> >
> >   When debugging in the reverse direction, gdb will work in replay mode as
> >   long as the execution log includes the record for the previous instruction;
> >   otherwise, it will work in record mode, if the platform supports reverse
> >   execution, or stop if not.
> >
> > [1] - https://sourceware.org/gdb/onlinedocs/gdb/Process-Record-and-
> Replay.html#Process-Record-and-Replay
> >
> > I think the "If the platform supports reverse execution" part is talking
> > about when the remote target supports reverse debugging directly,
> > like e.g., Qemu / Simics / VMWare(?).
> >
> > But then it seems confusing to call reverse stepping "record mode",
> > as in "if it's rewinding time, what it is recording??".

It looks like "replay" mode means that GDB is executing from its (own)
execution log; "record" mode means that GDB is extending its execution
log.

Traditionally, GDB records when stepping forward from the end of the
execution log.  On targets that support native reverse-stepping, however,
GDB is able to extend its execution log also when stepping backward from
the beginning of GDB's execution log.

This doesn't work with record btrace.

Regards,
Markus.

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