[Bug 1877688] Re: 9p virtfs device reports error when opening certain files

[Christian Schoenebeck <1877688@bugs.launchpad.net>]

The following patch should fix this bug for the kvm backend (not for the
XEN backend yet).

Please let me know if it fixes this bug for you.

** Patch added: "bug1877688_kvm_fix.patch"
   https://bugs.launchpad.net/qemu/+bug/1877688/+attachment/5369130/+files/bug1877688_kvm_fix.patch

** Changed in: qemu
       Status: New => In Progress

** Changed in: qemu
     Assignee: (unassigned) => Christian Schoenebeck (schoenebeck)

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https://bugs.launchpad.net/bugs/1877688

Title:
  9p virtfs device reports error when opening certain files

Status in QEMU:
  In Progress

Bug description:
  Reading certain files on a 9p mounted FS produces this error message:

  qemu-system-x86_64: VirtFS reply type 117 needs 12 bytes, buffer has
  12, less than minimum

  After this error message is generated, further accesses to the 9p FS
  hangs whatever tries to access it. The Arch Linux guest system is
  otherwise usable. This happens with QEMU 5.0.0 and guest kernel
  version 5.6.11, hosted on an Arch Linux distro. I use the following
  command to launch QEMU:

  exec qemu-system-x86_64 -enable-kvm -display gtk -vga virtio -cpu host
  -m 4G -netdev tap,ifname=vmtap0,id=vn0,script=no,downscript=no -device
  virtio-net-pci,netdev=vn0 -kernel kernel.img -drive
  file=file.img,format=raw,if=virtio -virtfs
  local,path=mnt,mount_tag=host0,security_model=passthrough,id=host0
  -append "console=ttyS0 root=/dev/vda rw"

  There's nothing relevant in the guest kernel logs as far as I'm aware
  of with loglevel set to 7.

  I tracked down the issue to readv() with a small buffer(<=12 bytes)
  and then a large buffer(>= 1024 bytes). A C program is provided to
  trigger this behavior.

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