Re: Best approach for supporting snapshots for QEMU's gdbstub?

[Alex Bennée]

Luis Machado <luis.machado@linaro.org> writes:

> Hi,
>
> On 5/14/21 1:06 PM, Alex Bennée wrote:
>> Hi,
>> I've been playing around with QEMU's reverse debugging support which
>> I have working with Pavel's latest patches for supporting virtio with
>> record/replay. Once you get the right command line it works well enough
>> although currently each step backwards requires replaying the entire
>> execution history until you get to the right point.
>> QEMU can quite easily snapshot the entire VM state so I was looking
>> to
>> see what the best way to integrate this would be. As far as I can tell
>> there are two interfaces gdb supports: bookmarks and checkpoints.
>> As far as I can tell bookmarks where added as part of GDB's reverse
>> debugging support but attempting to use them from the gdbstub reports:
>>    (gdb) bookmark
>>    You can't do that when your target is `remote'
>> so I guess that would need an extension to the stub protocol to
>> support?
>> 
>
> Right. We don't support reverse step/next/continue for remote targets.
> I think this would be the most appropriate way to implement this
> feature in GDB. But it is not trivial.

You do because ";ReverseStep+;ReverseContinue+" is part of the gdbstub
negotiation handshake.

Out of interest how is rr implemented? It presents a gdb interface so I
thought it was some implemented using some remote magic.

<snip>

>> We could of course just add a custom monitor command like the
>> qemu.sstep= command which could be used manually. However that would be
>> a QEMU gdbstub specific approach.
>
> That would be an easy and quick way to allow GDB to control things in
> QEMU, but I wouldn't say it is the best. Monitor commands are
> basically a bypass of the RSP where GDB sends/receives commands
> to/from the remote target.

We have some underlying commands we can set via the monitor including:

  monitor info replay
  monitor replay_seek <N>
  monitor replay_break <N>

>
>> The other thing would be to be more intelligent on QEMU's side and
>> save
>> snapshots each time we hit an event, for example each time we hit a
>> given breakpoint. However I do worry that might lead to snapshots
>> growing quite quickly.
>
> GDB would need to be aware of such snapshots for them to be useful.
> Otherwise GDB wouldn't be able to use them to restore state.

What does GDB need to know about them? Does it include something like
the icount at a particular point.

I'm curious at how a break and reverse-continue is meant to work if that
breakpoint is hit multiple times from the start of a run. You need to
know if the last time you hit a particular breakpoint was in fact the
last time before where the user was when they hit reverse-continue.

>
>> Any thoughts/suggestions?
>> 


-- 
Alex Bennée