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Re: [PATCH v3 09/15] replay: implement replay-seek command


From: Pavel Dovgalyuk
Subject: Re: [PATCH v3 09/15] replay: implement replay-seek command
Date: Tue, 8 Sep 2020 10:44:15 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 07.09.2020 19:25, Alex Bennée wrote:

Pavel Dovgalyuk <pavel.dovgalyuk@ispras.ru> writes:

On 07.09.2020 17:59, Alex Bennée wrote:

Pavel Dovgalyuk <pavel.dovgalyuk@ispras.ru> writes:

On 07.09.2020 15:58, Alex Bennée wrote:

Pavel Dovgalyuk <pavel.dovgalyuk@ispras.ru> writes:

From: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>

This patch adds hmp/qmp commands replay_seek/replay-seek that proceed
the execution to the specified instruction count.
The command automatically loads nearest snapshot and replays the execution
to find the desired instruction count.

Should there be an initial snapshot created at instruction 0? Using a
separate monitor channel:

Right, you can't go to the prior state, when there is no preceding
snapshot available.

It seems creating an initial snapshot automatically would be more user

Please take a look at 'Snapshotting' section of docs/replay.txt.
Reverse debugging is considered to be run with disk image (overlay)
and rrsnapshot option of icount, which allows creating an initial
VM snapshot.

Given that I'm using the block device purely for VM snapshots I think it
would be useful to document the minimal "no disk" approach - i.e. where
the disk is only used for record/replay.

However I'm still having trouble. I can record the trace with:

   ./qemu-system-aarch64 -cpu cortex-a53 -display none -serial stdio \
     -machine virt -kernel zephyr.elf -net none \
     -icount 
shift=6,align=off,sleep=off,rr=record,rrfile=record.out,rrsnapshot=rrstart  \
     -drive file=record.qcow2,if=none,id=rr \
     -monitor telnet:127.0.0.1:4444 -S

Can you provide your zephyr.elf image?


which shows:

   (qemu) info snapshots
   info snapshots
   List of snapshots present on all disks:
   ID        TAG               VM SIZE                DATE     VM CLOCK     
ICOUNT
   --        rrstart           653 KiB 2020-09-07 17:12:42 00:00:00.000         
 0

but do I need a whole separate overlay in the replay case? I thought
supplying snapshot to the drive would prevent the replay case
overwriting what has been recorded but with:

     -icount shift=6,align=off,sleep=off,rr=replay,rrfile=record.out \
     -drive file=record.qcow2,if=none,id=rr,snapshot

When you provide qcow2 (overlay or not) for snapshotting, you don't need any 'snapshot' option on drive.


but I get:

   (qemu) info snapshots
   info snapshots
   There is no snapshot available.

so if I drop the ,snapshot from the line I can at least see the snapshot
but continue doesn't seem to work:

   (qemu) info snapshots
   info snapshots
   List of snapshots present on all disks:
   ID        TAG               VM SIZE                DATE     VM CLOCK     
ICOUNT
   --        rrstart           653 KiB 2020-09-07 17:12:42 00:00:00.000         
 0
   (qemu) replay_break 190505
   replay_break 190505
   (qemu) c
   c
   (qemu) info replay
   info replay
   Replaying execution 'record.out': instruction count = 0

It seems, that replay hangs. Can you try removing '-S' in record command line?

   (qemu)

If I manually loadvm then we get somewhere but replay_seek breaks:

   (qemu) loadvm rrstart
   loadvm rrstart
   (qemu) info replay
   info replay
   Replaying execution 'record.out': instruction count = 190505
   (qemu) replay_seek 190000
   replay_seek 190000
   snapshotting is disabled

with a crash:

   ./qemu-system-aarch64 -cpu cortex-a53 -display none -serial stdio -machine 
virt -kernel zephyr.elf -net none -icount 
shift=6,align=off,sleep=off,rr=replay,rrfile=record.out
  -drive file=record.qcow2,if=none,id=rr -monitor telnet:127.0.0.1:4444 -S
*** Booting Zephyr OS build zephyr-v2.3.0-1183-ge5628ad0faf3  ***
Hello World! qemu_cortex_a53
free(): invalid pointer
fish: “./qemu-system-aarch64 -cpu cort…” terminated by signal SIGABRT (Abort)



friendly? What can you do to trigger a snapshot, say for example on a
gdb connect?

This makes sense when executing with temporary overlay, thanks.





Pavel Dovgalyuk



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