Re: [Qemu-devel] [RFC PATCH 00/17] reverse debugging

[Pavel Dovgalyuk]

> From: Ciro Santilli [mailto:address@hidden
> On Wed, Apr 25, 2018 at 1:45 PM, Pavel Dovgalyuk
> <address@hidden> 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.
> >
> 
> Hi Pavel,
> 
> 1)
> 
> Can you provide more details on how to run the reverse debugging? In
> particular how to take the checkpoint?

There is some information in docs/replay.txt, but I guess, that I can give some 
more.

> 
> My test setup is described in detail at:
> https://github.com/cirosantilli/qemu-test/tree/8127452e5685ed233dc7357a1fe34b7a2d173480
> command "x86_64/reverse-debug".
> 
> Here are the actual commands:
> 
> #!/usr/bin/env bash
> set -eu
> dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)/.."
> cmd="\
> time \
> ./x86_64-softmmu/qemu-system-x86_64 \
> -M pc \
> -append 'root=/dev/sda console=ttyS0 nokaslr printk.time=y -
> lkmc_eval=\"/rand_check.out;/sbin/ifup -a;wget -S
> google.com;/poweroff.out;\"' \
> -kernel '${dir}/out/x86_64/buildroot/images/bzImage' \
> -nographic \
> -serial mon:stdio \
> -monitor telnet::45454,server,nowait \
> \
> -drive 
> file='${dir}/out/x86_64/buildroot/images/rootfs.ext2.qcow2,if=none,id=img-
> direct,format=qcow2,snapshot'

The main thing for reverse debugging is snapshotting.
Therefore you should have an image that does not use temporary overlay file 
(snapshot option).
I'm using the following command line for record:

rm ./images/xp.ovl
# create overlay to avoid modifying the original image
./bin/qemu-img create -f qcow2 -b xp.qcow2 ./images/xp.ovl
./bin/qemu-system-i386 \
# This is workaround for XP. I wonder is it needed for the current version or 
not.
 -global apic-common.vapic=off \
# using newly created overlay instead of the original image
# rrsnapshot creates the snapshot at the start
 -icount shift=7,rr=record,rrfile=xp.replay,rrsnapshot=init -drive 
file=./images/xp.ovl,if=none,id=img-direct \
 -drive driver=blkreplay,if=none,image=img-direct,id=img-replay -device 
ide-hd,drive=img-replay -net none -m 256M -monitor stdio

While recording I can create some snapshots with savevm.
Command line for replaying differs only in "rr" option. rrsnapshot there loads 
the initial snapshot.
Any of the previously created snapshots may be specified.
You can also create new snapshots while replaying.


> \
> -drive driver=blkreplay,if=none,image=img-direct,id=img-blkreplay \
> -device ide-hd,drive=img-blkreplay \
> \
> -netdev user,id=net1 \
> -device rtl8139,netdev=net1 \
> -object filter-replay,id=replay,netdev=net1 \
> "
> cmd="${cmd} $@"
> echo "$cmd"
> eval "$cmd -icount 'shift=7,rr=record,rrfile=replay.bin'"
> eval "$cmd -icount 'shift=7,rr=replay,rrfile=replay.bin' -S -s"
> 
> Then I take a snapshot right at the beginning of the execution:
> 
> telnet 45454
> savevm a
> 
> And on another shell:
> 
> /data/git/linux-kernel-module-cheat/out/x86_64/buildroot/host/usr/bin/x86_64-linux-gdb
> \
> -q \
> -ex 'file vmlinux' \
> -ex 'target remote localhost:1234' \
> -ex 'break start_kernel' \
> -ex 'continue' \
> 
> But now if I try on GDB:
> 
> next
> next
> next
> reverse-continue
> 
> hoping to go back to start_kernel, but nothing happens.

Yes, because you are missing your snapshot, that was actually created in the 
temporary overlay.

> Same behavior if I take the snapshot after reaching start_kernel instead.
> 
> 2)
> 
> I wonder if it would be possible to expose checkpoint taking through
> GDB example via:
> https://sourceware.org/gdb/onlinedocs/gdb/Checkpoint_002fRestart.html

We'll check this out.

> Or some other more convenient checkpoint generation method, e.g.
> automatically take checkpoints every N instructions.

We implemented 'taking snapshots every N seconds', but I'll prefer to submit
it later, after approving the main idea.

> > 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-180207

Pavel Dovgalyuk