Simics & reverse execution

[Greg Law]

On 09/07/2009 09:24 AM, Jakob Engblom wrote:
>>> going to be painful.  Time really becomes very pervasive once
>>> you start using it in one place...
>>
>> Intriguing.  In what ways?
>>
>> We currently have a somewhat baroque bunch of python that wraps up
>> gdb to enable our reverse debugging commands on users system where
>> they've an older version of gdb installed.  This includes some
>> commands to display the current time and jump to an arbitrary time
>> (where time is a scalar 64-bit integer).  The rest of gdb remains
>> blissfully unaware (obviously, since it's unpatched).
>>
>> I can see how in general "teaching" the concept of time to gdb
>> would be a huge job.  But I don't think a command to jump around
>> arbitrarily and another to display the "current" time needs to
>> teach anything about time to the rest of gdb.
>
> Think about the cross-product with thread handling.  If you use time
> there, you need to determine how to handle times for threads. One
> timeline per thread? Or a global system time?

Oh right, yes, I was assuming global.

If time were per thread, then yes, I can see how that would quickly get
very complicated.

>
> You also really like to have time breakpoints in the system, to
> allow you to do things like "run this system for 6.132 seconds and
> then stop" (typical operation we do when skipping a boot sequence or
> we have run a workload several times and know when itneresting
> things should start to happen).

I would argue that's a future feature.  Time-based breakpoints would
seem a very nice thing to have, but a system which had get/set time
commands but no time breakpoints would be a lot more more useful than
one with neither.

>
> Also, how should this interact with non-stop debug? And how should
> you handle multiple active processor cores running parallel threads?

I must confess, I haven't given much thought at all to the interaction
of non-stop debug with reverse debugging in general.  I haven't been
following too closely, but aren't they currently incompatible anyway?
(something to do with displaced breakpoints not working well with
reverse).  But I don't see how it's any worse than the bookmarks case.
In fact, I don't see how any of it differs from use with bookmarks.  All
we're really talking about is a different way to describe a point in
history.

Again, you could go a lot further than I'm proposing right now.  But
that's not to say you need to for this stuff be useful.

Greg

-- 
Greg Law, Undo Software                       http://undo-software.com/