[Qemu-devel] [Bug 1672365] Re: nested 9pfs read fail

[Leo Gaspard]

Oh, I forgot to mention: it first worked for some time, then in the
middle of a shell session running over a screen
/var/lib/vm/consoles/nginx/screen from the outer VM (socat-linked to
/var/lib/vm/consoles/nginx/socket.unix to provide a predictable pty
link), the 9pfs stopped returning any data, and it didn't go away after
a reboot of the inner VM, as it then no longer managed to boot.

I was unfortunately unable to identify exactly which operation caused
the thing to "stop working", but I'd say it is due to zsh's path-full
autocompletion in paths including directories with ~700 entries, without
being certain of that.

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
https://bugs.launchpad.net/bugs/1672365

Title:
  nested 9pfs read fail

Status in QEMU:
  New

Bug description:
  tl;dr: A virtfs read fails. The init being on this virtfs (mounted by
  the initrd), the linux kernel guest is unable to boot, and kernel
  panics. The fact that qemu still takes 100%cpu after the kernel panic
  makes me think it's a qemu bug.

  Here is the setup (some hashes replaced with "..."):
   * A (NixOS) host system, with /nix/store as a btrfs on lvm on cryptsetup
   * Running a qemu-kvm NixOS guest, with /nix/.ro-store as a virtfs mapping to 
host /nix/store:
  ```
  exec /nix/store/...-qemu-x86-only-for-vm-tests-2.8.0/bin/qemu-kvm \
      -name test \
      -m 8192 \
      -cpu kvm64 \
      -net nic,vlan=0,model=virtio -net user,vlan=0 \
      -virtfs local,path=/nix/store,security_model=none,mount_tag=store \
      -virtfs 
local,path=/tmp/nix-vm..../xchg,security_model=none,mount_tag=xchg \
      -virtfs 
local,path=/tmp/nix-vm..../xchg,security_model=none,mount_tag=shared \
      -drive 
index=0,id=drive1,file=/home/ekleog/nixos/test.qcow2,if=virtio,cache=writeback,werror=report
 \
  -kernel /nix/store/...-nixos-system-test-17.09.git.deee8da/kernel \
  -initrd /nix/store/...-nixos-system-test-17.09.git.deee8da/initrd \
  -append "$(cat 
/nix/store/...-nixos-system-test-17.09.git.deee8da/kernel-params) 
init=/nix/store/...-nixos-system-test-17.09.git.deee8da/init 
regInfo=/nix/store/...-reginfo" \
      -vga std -usbdevice tablet
  ```
   * With /nix/store as an overlayfs between /nix/.ro-store and /nix/.rw-store
   * Running a qemu-kvm NixOS guest, with /nix/.ro-store as a virtfs mapping to 
host /nix/store/...-vm-nginx-store:
  ```
  /nix/store/...-qemu-2.8.0/bin/qemu-kvm \
    -name nginx -m 128 -smp 2 -cpu kvm64 \
    -nographic -serial 
unix:"/var/lib/vm/consoles/nginx/socket.unix",server,nowait \
    -drive file="/var/lib/vm/images/nginx.img",if=virtio,media=disk \
    -virtfs 
local,path="/nix/store/...-vm-nginx-store",security_model=none,mount_tag=store \
    -netdev type=tap,id=net0,ifname=vm-nginx,script=no,dscript=no -device 
virtio-net-pci,netdev=net0,mac=56:00:00:00:00:01 \
    -kernel /nix/store/...-nixos-system-nginx-17.09.git.deee8da/kernel \
    -initrd /nix/store/...-nixos-system-nginx-17.09.git.deee8da/initrd \
    -append "$(cat 
/nix/store/...-nixos-system-nginx-17.09.git.deee8da/kernel-params) 
init=/nix/store/...-nixos-system-nginx-17.09.git.deee8da/init console=ttyS0 
boot.shell_on_fail" \
    -virtfs 
local,path="/var/lib/vm/persist/nginx/home",security_model=mapped-xattr,mount_tag="shared1"
 \
    -virtfs 
local,path="/var/lib",security_model=mapped-xattr,mount_tag="shared2" \
    -virtfs local,path="/tmp",security_model=mapped-xattr,mount_tag="shared3"
  ```
   * With /nix/store as an overlayfs between /nix/.ro-store and /nix/.rw-store
   * With init in /nix/store

  What happens is that at boot time the inner VM doesn't manage to read
  the init script after the initrd has mounted the 9pfs and overlayfs.

  What makes me think it's a qemu bug is that htop in the outer VM shows
  the inner VM's qemu as using 100% cpu, despite the fact the kernel
  it's running is under kernel panic, thus doing nothing.

  What do you think about this?

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1672365/+subscriptions