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

[Leo Gaspard]

Hmm, actually it looks like a kernel in kernel panic always takes 100%
CPU (just got another unrelated one), so I guess it's not necessarily a
qemu bug but can arise from anywhere in the stack. I'm marking the bug
as invalid as a consequence.

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

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https://bugs.launchpad.net/bugs/1672365

Title:
  nested 9pfs read fail

Status in QEMU:
  Invalid

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?

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