RE: [for-5.0 PATCH 00/11] Support for reverse debugging with GDB

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

> From: Alex Bennée [mailto:alex.bennee@linaro.org]
> Pavel Dovgalyuk <dovgaluk@ispras.ru> writes:
> 
> >> From: Alex Bennée [mailto:alex.bennee@linaro.org]
> >> Pavel Dovgalyuk <pavel.dovgaluk@gmail.com> writes:
> >>
> >> > 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
> >> >
> >> > The patches are available in the repository:
> >> > https://github.com/ispras/qemu/tree/rr-191223
> >>
> >> So I tried with your additional patch. Launching QEMU as:
> >>
> >>   ./aarch64-softmmu//qemu-system-aarch64 -monitor none \
> >>      -display none -M virt -cpu max -display none \
> >>      -semihosting-config enable=on \
> >>      -kernel ./tests/tcg/aarch64-softmmu/memory \
> >>      -icount shift=5,rr=replay,rrfile=record.bin \
> >>      -s -S -d trace:gdbstub\*
> >>
> >> And gdb:
> >>
> >>   gdb tests/tcg/aarch64-softmmu/memory \
> >>     -ex "target remote localhost:1234"
> >>
> >> I get the following log:
> >>
> >>   (gdb) x/3i $pc
> >>   => 0x400037b0 <__start>:        adr     x0, 0x40003000 <vector_table>
> >>      0x400037b4 <__start+4>:      msr     vbar_el1, x0
> >>      0x400037b8 <__start+8>:      adrp    x0, 0x40200000
> >>   (gdb) p/x $x0
> >>   $1 = 0x0
> >>   (gdb) si
> >>   92              msr     vbar_el1, x0
> >>   (gdb) p/x $x0
> >>   $2 = 0x40003000
> >>   (gdb) rsi
> >>   warning: Remote failure reply: E14
> >>
> >>   Program stopped.
> >>   __start () at /home/alex.bennee/lsrc/qemu.git/tests/tcg/aarch64/system/boot.S:92
> >>   92              msr     vbar_el1, x0
> >>   (gdb) p/x $x0
> >>   $3 = 0x40003000
> >>
> >> So it doesn't seem to be working.
> >
> > That's ok, you'll need at least one VM snapshot available to recover the initial VM state.
> > Try changing the command lines in the following way:
> >
> > First, create empty.qcow2 which will be used for saving the snapshots.
> > Then record with initial snapshot and attached empty.qcow2:
> >
> >    ./aarch64-softmmu//qemu-system-aarch64 -monitor none \
> >       -display none -M virt -cpu max \
> >       -kernel ./tests/tcg/aarch64-softmmu/memory \
> >       -icount shift=5,rr=record,rrfile=record.bin,rrsnapshot=init \
> >       -drive file=empty.qcow2
> 
> ./aarch64-softmmu//qemu-system-aarch64 -monitor none -display none -M virt -cpu max -display
> none -semihosting-config enable=on -kernel ./tests/tcg/aarch64-softmmu/memory -icount
> shift=5,rr=record,rrfile=record.bin,rrsnapshot=init -drive file=empty.qcow2


> qemu-system-aarch64: invalid accelerator kvm
> qemu-system-aarch64: falling back to tcg
> qemu-system-aarch64: The qcow format used by node '#block163' does not support live migration
> qemu-system-aarch64: Could not create snapshot for icount record

It seems that you have some problems with your disk image. Is it qcow2 or just qcow?

> For this testcase semihosting in just a convenient output device (in
> lieu of a serial device). 

I tried this test kernel with your options and everything was ok.

> We probably need to come up with a strategy on
> how we handle all these devices otherwise we will end up with a random
> selection of hardware combinations which work.

All correctly implemented virtual hardware should support record/replay.
But real semihosting (like file IO) should not, because it provides
untracked virtual machine inputs.

Pavel Dovgalyuk