Re: [Qemu-devel] [PATCH v4 00/19] reverse debugging

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

> From: Alex Bennée [mailto:alex.bennee@linaro.org]
> Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru> writes:
> 
> > Ping?
> 
> I started having a look but I ran into this straight away. First I
> recorded a boot of the kernel:
> 
>   ./aarch64-softmmu/qemu-system-aarch64 -machine virt,graphics=on,gic-
> version=3,virtualization=on -cpu cortex-a53 --serial mon:stdio -display none -kernel
> ../images/aarch64-current-linux-initrd-guest.img -icount shift=7,rr=record,rrfile=replay.bin
> 
> Then played back:
> 
>   ./aarch64-softmmu/qemu-system-aarch64 -machine virt,graphics=on,gic-
> version=3,virtualization=on -cpu cortex-a53 --serial mon:stdio -display none -kernel
> ../images/aarch64-current-linux-initrd-guest.img -icount shift=7,rr=replay,rrfile=replay.bin -
> s -S

This looks ok, but...

> And did the following on gdb:
> 
> (gdb) i
> 0x0000000040000004 in ?? ()
> => 0x40000004:  mov     x1, xzr
>    0x40000008:  mov     x2, xzr
>    0x4000000c:  mov     x3, xzr
> (gdb)
> 0x0000000040000008 in ?? ()
> => 0x40000008:  mov     x2, xzr
>    0x4000000c:  mov     x3, xzr
>    0x40000010:  ldr     x4, 0x40000020
> (gdb)
> 0x000000004000000c in ?? ()
> => 0x4000000c:  mov     x3, xzr
>    0x40000010:  ldr     x4, 0x40000020
>    0x40000014:  br      x4
> (gdb)
> 0x0000000040000010 in ?? ()
> => 0x40000010:  ldr     x4, 0x40000020
>    0x40000014:  br      x4
>    0x40000018:  .inst   0x44000000 ; undefined
> (gdb)
> 0x0000000040000014 in ?? ()
> => 0x40000014:  br      x4
>    0x40000018:  .inst   0x44000000 ; undefined
>    0x4000001c:  .inst   0x00000000 ; undefined
> (gdb) p/x $x4
> $1 = 0x40080000
> (gdb) reverse-stepi
> warning: Remote failure reply: E14
> 
> Surely this is the simple case and doesn't require any snapshots for
> block devices as there are none. Am I missing something?

Reverse debugging requires the snapshotting. QEMU can't revert the VM state without the snapshots.
You can try adding an empty qcow2 image to allow snapshotting there.

Pavel Dovgalyuk