Qemu VirtFS mounts are not accessible after resuming guest from hibernation

[Daniel Harding]

Host OS: Funtoo Linux
Host Kernel: 4.7.4-gentoo
Qemu Version: 2.7.0
Guest OS: Ubuntu 14.04
Guest Kernel: reproduced with both 4.4.0-42-generic and 3.13.0-98-generic

Qemu command line:

qemu-system-x86_64 \
    -machine type=pc,accel=kvm \
    -cpu host \
    -smp 3 \
    -m 8G \
    -netdev bridge,id=hn0,vhost=on \
    -device virtio-net-pci,netdev=hn0,mac=52:54:fa:70:35:f7 \
    -drive file=/dev/mapper/vg-ubuntu,format=raw,if=virtio,cache=none,discard=on \
    -virtfs local,path=/home/dharding/code,security_model=passthrough,mount_tag=code \
    -virtfs local,path=/home/dharding/xfer,security_model=passthrough,mount_tag=xfer \
    -display gtk

Relevant lines from guest /etc/fstab:
code /home/dharding/code 9p trans=virtio,version=9p2000.L,msize=262144,_netdev 0 0
xfer /home/dharding/xfer 9p trans=virtio,version=9p2000.L,msize=262144,_netdev 0 0

Steps to reproduce:
- start qemu using the above command line
- in the guest, run "sudo pm-hibernate"
- after qemu exits, run again using the same command line
- once the guest resumes from hibernation, run "ls /home/dharding/code"
- the ls command will hang forever

The ls call stack is:

[<ffffffffc00743a0>] p9_client_rpc+0x110/0x460 [9pnet]
[<ffffffffc0076a50>] p9_client_getattr_dotl+0x60/0x160 [9pnet]
[<ffffffffc009ef77>] v9fs_vfs_getattr_dotl+0x47/0xa0 [9p]
[<ffffffff81202a5c>] vfs_getattr_nosec+0x2c/0x40
[<ffffffff81202b26>] vfs_getattr+0x26/0x30
[<ffffffff81202bf5>] vfs_fstatat+0x65/0xa0
[<ffffffff8120306f>] SYSC_newstat+0x1f/0x40
[<ffffffff812032be>] SyS_newstat+0xe/0x10
[<ffffffff817fa4f6>] entry_SYSCALL_64_fastpath+0x16/0x75

[Greg Kurz (gkurz)]

The root cause lies in the 9pnet_virtio driver in the guest kernel: it does not support suspend/hibernation...