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

["Alex Bennée" <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

I don't know if this is down to the current state of the tree but I
haven't been able to get it working.

>
> After that you can replay the execution with the support of reverse debugging:
>
>    ./aarch64-softmmu//qemu-system-aarch64 -monitor none \
>       -display none -M virt -cpu max \
>       -kernel ./tests/tcg/aarch64-softmmu/memory \
>       -icount shift=5,rr=replay,rrfile=record.bin,rrsnapshot=init \
>       -drive file=empty.qcow2
>
> I removed "-semihosting-config enable=on", because I'm not sure what is it for.
> It may break the replay, because there is no implementation of
> semihosting input record/replay.

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

>
> Pavel Dovgalyuk


-- 
Alex Bennée