Re: [Qemu-devel] [PATCH v3 00/19] reverse debugging

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Ciro Santilli [mailto:ciro.santilli@gmail.com]
> On Wed, May 23, 2018 at 2:28 PM, Pavel Dovgalyuk <dovgaluk@ispras.ru> wrote:
> >> From: Ciro Santilli [mailto:ciro.santilli@gmail.com]
> >> On Wed, May 23, 2018 at 7:49 AM, Pavel Dovgalyuk
> >> <Pavel.Dovgaluk@ispras.ru> wrote:
> >> > GDB remote protocol supports reverse debugging of the targets.
> >> > It includes 'reverse step' and 'reverse continue' operations.
> >> > The first one finds the previous step of the execution,
> >> > and the second one is intended to stop at the last breakpoint that
> >> > would happen when the program is executed normally.
> >> >
> >> > Reverse debugging is possible in the replay mode, when at least
> >> > one snapshot was created at the record or replay phase.
> >> > QEMU can use these snapshots for travelling back in time with GDB.
> >> >
> >> > Running the execution in replay mode allows using GDB reverse debugging
> >> > commands:
> >> >  - reverse-stepi (or rsi): Steps one instruction to the past.
> >> >    QEMU loads on of the prior snapshots and proceeds to the desired
> >> >    instruction forward. When that step is reaches, execution stops.
> >> >  - reverse-continue (or rc): Runs execution "backwards".
> >> >    QEMU tries to find breakpoint or watchpoint by loaded prior snapshot
> >> >    and replaying the execution. Then QEMU loads snapshots again and
> >> >    replays to the latest breakpoint. When there are no breakpoints in
> >> >    the examined section of the execution, QEMU finds one more snapshot
> >> >    and tries again. After the first snapshot is processed, execution
> >> >    stops at this snapshot.
> >> >
> >> > The set of patches include the following modifications:
> >> >  - gdbstub update for reverse debugging support
> >> >  - functions that automatically perform reverse step and reverse
> >> >    continue operations
> >> >  - hmp/qmp commands for manipulating the replay process
> >> >  - improvement of the snapshotting for saving the execution step
> >> >    in the snapshot parameters
> >> >  - other record/replay fixes
> >> >
> >> > The patches are available in the repository:
> >> > https://github.com/ispras/qemu/tree/rr-180428
> >> >
> >>
> >> This branch appears to contain one month old commits, is it the correct one?
> >
> > Right.
> > There were no significant changes except the fix which was already queued by Paolo.
> > As soon as it is upstreamed, I'll update the branch.
> >
> 
> OK.
> 
> At the current branch 6b23df0d0ca0e5e999cd12af2b18b2a95faeb421 still
> observe the same behaviour as mentioned at:
> https://lists.gnu.org/archive/html/qemu-devel/2018-04/msg04669.html
> 
> If I try to savevm during the debug replay to speedup up reverse
> execution, QEMU hangs.

This behavior was fixes by a separate patch of PS/2 controller.
Here is the branch including it: https://github.com/ispras/qemu/tree/rr-180524

> Have you managed to reproduce that? Or is this not an intended use
> case, i.e. only savevm during record is supported?

Yes, I reproduced and fixed it.
savevm should work correctly during both record and replay.

> Am I correct to understand that being able to do savevms in the middle
> of a long execution is the critical feature that this adds? Otherwise
> we are essentially replaying from the initial snapshot every time, so
> we might as well just restart a new replay, is that true?

We can start replay from any of the snapshots creating during the record and replay.

Pavel Dovgalyuk