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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