Best approach for supporting snapshots for QEMU's gdbstub?

[Luis Machado]

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.

> The other option I found was checkpoints which seem to predate support
> for reverse debugging. However:
> 
>    (gdb) checkpoint
>    checkpoint: can't find fork function in inferior.
> 
> I couldn't tell what feature needs to be negotiated but I suspect it's
> something like fork-events if the checkpoint mechanism is designed for
> user space with a fork/freeze approach.

Checkpoints are an old mechanism for saving a snapshot from a process, 
but they don't support threaded inferiors and they need to be able to 
call fork.

> 
> 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.

> 
> 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.

> 
> Any thoughts/suggestions?
>