[Qemu-devel] [Bug 1839807] Re: Snapshots freeze guest Sabrelite IMX.6 board

[Peter Maydell]

The underlying cause of this is that we're not migrating the Secure
banked cp15 register contents. So boards which don't enable TrustZone or
where the guest runs in the NonSecure state (like the virt board, etc)
can save/restore fine, but since the imx6 happens to run the guest in
the Secure state it gets hit by the bug and its MMU setup etc is
completely broken on restore.

This is a bug I knew about (I mentioned it in LP:1739378 comment #5),
but I've forgotten the detail of why it happens and don't seem to have
written it down, so I'm going to have to think it through again...


** Changed in: qemu
       Status: New => Confirmed

** Tags added: arm

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https://bugs.launchpad.net/bugs/1839807

Title:
  Snapshots freeze guest Sabrelite IMX.6 board

Status in QEMU:
  Confirmed

Bug description:
  Hello,

  I'm trying to take and restore  a snapshot with the whole system state of the 
Sabrelite IMX.6 board running on QEMU with commands savevm/loadvm.
  It seems that I am able to take a snapshot but loading the snapshot fails.

  For comparison I checked out snapshots on 32bit ARM Virt with Debian as well 
as on the Versatilepb board with a bare metal application and it works fine.
  The problem occurs only with that one particular board.

  My environment is:
  Ubuntu 18.04
  QEMU 3.0.1 (I see the same issue in QEMU 4.0.0 as well)
  The kernel and device tree used for the board was 5.1.14 version from 
kernel.org

  The file system was build from imx_v6_v7_defconfig config in buildroot
  as and sd card image.

  Problem:

  Loading snapshot stops the whole machine and it's impossible to resume
  it.

  Steps to reproduce problem:

  1.      I converted the sdcard.img built from the buildroot to qcow2
  using command qemu-img convert -f raw -O qcow2 sdcard.img
  sdcard.qcow2, since the raw doesn't support snapshots.

  2.      I start QEMU with a command
  ./arm-softmmu/qemu-system-arm -m 512 -M sabrelite -kernel zImage -append 
"rootfstype=ext4 root=/dev/mmcblk2p2 rw rootwait" -rtc base=localtime,clock=vm 
-dtb imx6dl-sabresd.dtb -drive file=sdcard.qcow2,index=2,format=qcow2,id=mycard 
-device sd-card,drive=mycard -nographic -net nic -net user

  3.      I run a simple program which print characters to the console
  in the background and add some files in user directory, to differ from
  original image.

  4.      I switch to QEMU monitor, and type “savevm <name>”.
  When I type “info snapshots”, the snapshot is listed.
  So I assume it was saved correctly.

  5.      Then I switch back to Linux console from monitor, remove the
  added files and stop the background printing process.

  6.      I switch back to monitor and I'm trying now to load the
  snapshot by “loadvm <name>” command.

  That’s where the problem occurs. QEMU stops and I can't switch back from 
monitor to Linux.
  Typing “cont” doesn’t help.
  It seems like the simulation has freezed. CPU usage on my Laptop machine 
equals 100% until I exit QEMU.

  
  What’s interesting when I exit the QEMU and then start it again the Linux 
boots and after it reaches the command prompt I can see the files which were 
removed after saving the snapshot.

  It looks like loading the snapshots works for restoring disk space but
  it fails for restoring the running processes.

  Due to the answer on QEMU mailing list
  (https://lists.nongnu.org/archive/html/qemu-
  discuss/2019-08/msg00016.html) it is QEMUs bug.

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