Re: [Qemu-devel] [Bug 1224444] Re: virtio-serial loses writes when used over virtio-mmio

["Laszlo Ersek \(Red Hat\)" <lersek@redhat.com>]

On 09/17/13 10:09, Peter Maydell wrote:
> On 16 September 2013 17:13, Laszlo Ersek <lersek@redhat.com> wrote:
>> Hmmmm. Normally (as in, virtio-pci), when a VCPU thread (running KVM)
>> executes guest code that sends data to the host via virtio, KVM kicks
>> the "host notifier" eventfd.
> 
> What happens in the virtio-pci without eventfd case?
> (eg virtio-pci on a non-x86 host)

I'm confused. I think Anthony or Michael could answer better.

There's at least three cases here I guess (KVM + eventfd, KVM without
eventfd (enforceable eg. with the "ioeventfd" property for virtio
devices), and TCG). We're probably talking about the third case.

I think we end up in

  virtio_pci_config_ops.write == virtio_pci_config_write
    virtio_ioport_write()
      virtio_queue_notify()
        ... the "usual" stuff ...

As far as I know TCG supports exactly one VCPU thread but that's still
separate from the IO-thread. In that case the above could trigger the
problem similarly to virtio-mmio I guess...

I think we should debug into GLib, independently of virtio. What annoys
me mostly is the huge number of ppoll()s in Rich's trace between
connecting to the UNIX domain socket and actually checking it for
read-readiness. The fd in question should show up in the first ppoll()
after connect().

My email might not make any sense. Sorry.
Laszlo

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https://bugs.launchpad.net/bugs/1224444

Title:
  virtio-serial loses writes when used over virtio-mmio

Status in QEMU:
  New

Bug description:
  virtio-serial appears to lose writes, but only when used on top of
  virtio-mmio.  The scenario is this:

  /home/rjones/d/qemu/arm-softmmu/qemu-system-arm \
      -global virtio-blk-device.scsi=off \
      -nodefconfig \
      -nodefaults \
      -nographic \
      -M vexpress-a15 \
      -machine accel=kvm:tcg \
      -m 500 \
      -no-reboot \
      -kernel /home/rjones/d/libguestfs/tmp/.guestfs-1001/kernel.27944 \
      -dtb /home/rjones/d/libguestfs/tmp/.guestfs-1001/dtb.27944 \
      -initrd /home/rjones/d/libguestfs/tmp/.guestfs-1001/initrd.27944 \
      -device virtio-scsi-device,id=scsi \
      -drive file=/home/rjones/d/libguestfs/tmp/libguestfsLa9dE2/scratch.1,cache=unsafe,format=raw,id=hd0,if=none \
      -device scsi-hd,drive=hd0 \
      -drive file=/home/rjones/d/libguestfs/tmp/.guestfs-1001/root.27944,snapshot=on,id=appliance,cache=unsafe,if=none \
      -device scsi-hd,drive=appliance \
      -device virtio-serial-device \
      -serial stdio \
      -chardev socket,path=/home/rjones/d/libguestfs/tmp/libguestfsLa9dE2/guestfsd.sock,id=channel0 \
      -device virtserialport,chardev=channel0,name=org.libguestfs.channel.0 \
      -append 'panic=1 mem=500M console=ttyAMA0 udevtimeout=600 no_timer_check acpi=off printk.time=1 cgroup_disable=memory root=/dev/sdb selinux=0 guestfs_verbose=1 TERM=xterm-256color'

  After the guest starts up, a daemon writes 4 bytes to a virtio-serial
  socket.  The host side reads these 4 bytes correctly and writes a 64
  byte message.  The guest never sees this message.

  I enabled virtio-mmio debugging, and this is what is printed (## = my
  comment):

  ## guest opens the socket:
  trying to open virtio-serial channel '/dev/virtio-ports/org.libguestfs.channel.0'
  virtio_mmio: virtio_mmio_write offset 0x50 value 0x3
  opened the socket, sock = 3
  udevadm settle
  ## guest writes 4 bytes to the socket:
  virtio_mmio: virtio_mmio_write offset 0x50 value 0x5
  virtio_mmio: virtio_mmio setting IRQ 1
  virtio_mmio: virtio_mmio_read offset 0x60
  virtio_mmio: virtio_mmio_write offset 0x64 value 0x1
  virtio_mmio: virtio_mmio setting IRQ 0
  sent magic GUESTFS_LAUNCH_FLAG
  ## host reads 4 bytes successfully:
  main_loop libguestfs: recv_from_daemon: received GUESTFS_LAUNCH_FLAG
  libguestfs: [14605ms] appliance is up
  Guest launched OK.
  ## host writes 64 bytes to socket:
  libguestfs: writing the data to the socket (size = 64)
  waiting for next request
  libguestfs: data written OK
  ## hangs here forever with guest in read() call never receiving any data

  I am using qemu from git today (2d1fe1873a984).

  Some notes:

  - It's not 100% reproducible.  Sometimes everything works fine, although it fails "often" (eg > 2/3rds of the time).
  - KVM is being used.
  - We've long used virtio-serial on x86 and have never seen anything like this.

  This is what the output looks like when it doesn't fail:

  trying to open virtio-serial channel '/dev/virtio-ports/org.libguestfs.channel.0
  '
  virtio_mmio: virtio_mmio_write offset 0x50 value 0x3
  opened the socket, sock = 3
  udevadm settle
  virtio_mmio: virtio_mmio_write offset 0x50 value 0x5
  virtio_mmio: virtio_mmio setting IRQ 1
  virtio_mmio: virtio_mmio_read offset 0x60
  virtio_mmio: virtio_mmio_write offset 0x64 value 0x1
  virtio_mmio: virtio_mmio setting IRQ 0
  sent magic GUESTlibguestfs: recv_from_daemon: received GUESTFS_LAUNCH_FLAG
  libguestfs: [14710ms] appliance is up
  Guest launched OK.
  libguestfs: writing the data to the socket (size = 64)
  FS_LAUNCH_FLAG
  main_loop waiting for next request
  libguestfs: data written OK
  virtio_mmio: virtio_mmio_write offset 0x50 value 0x2
  virtio_mmio: virtio_mmio setting IRQ 1
  virtio_mmio: virtio_mmio setting IRQ 1
  virtio_mmio: virtio_mmio_write offset 0x50 value 0x2
  virtio_mmio: virtio_mmio_read offset 0x60
  virtio_mmio: virtio_mmio setting IRQ 1
  virtio_mmio: virtio_mmio_write offset 0x64 value 0x1
  virtio_mmio: virtio_mmio setting IRQ 0
  virtio_mmio: virtio_mmio_read offset 0x60
  virtio_mmio: virtio_mmio_write offset 0x64 value 0x0
  virtio_mmio: virtio_mmio setting IRQ 0
  virtio_mmio: virtio_mmio_read offset 0x60
  virtio_mmio: virtio_mmio_write offset 0x64 value 0x1
  [... more virtio-mmio lines omitted ...]
  virtio_mmio: virtio_mmio_write offset 0x64 value 0x0
  virtio_mmio: virtio_mmio setting IRQ 1
  virtio_mmio: virtio_mmio_read offset 0x60
  virtio_mmio: virtio_mmio_write offset 0x64 value 0x1
  virtio_mmio: virtio_mmio setting IRQ 0
  guestfsd: main_loop: new request, len 0x3c
  virtio_mmio: virtio_mmio_write offset 0x50 value 0x4
  0000: 20 00 f5 f5 00 00 00 04 00 00 00 d2 00 00 00 00 | ...............|virtio_mmio: virtio_mmio_write offset 0x50 value 0x2
  virtio_mmio: virtio_mmio setting IRQ 1

  virtio_mmio: virtio_mmio_read offset 0x60
  virtio_mmio: virtio_mmio_write offset 0x64 value 0x1
  virtio_mmio: virtio_mmio setting IRQ 0
  0010: 00 12 34 00 00 00 00 00 00 00 00 00 00 00 00 00 |..4.............|
  0020: 00 00 00 00 00 00 00 00 00 00 00 08 2f 64 65 76 |............/dev|
  0030: 2f 73 64 61 00 00 00 03 6d 62 72 00             |/sda....mbr.    |
  virtio_mmio: virtio_mmio_write offset 0x50 value 0x2
  virtio_mmio: virtio_mmio setting IRQ 1
  virtio_mmio: virtio_mmio_read offset 0x60
  virtio_mmio: virtio_mmio_write offset 0x64 value 0x1
  virtio_mmio: virtio_mmio setting IRQ 0
  virtio_mmio: virtio_mmio_write offset 0x50 value 0x2
  virtio_mmio: virtio_mmio setting IRQ 1
  virtio_mmio: virtio_mmio_read offset 0x60
  virtio_mmio: virtio_mmio_write offset 0x64 value 0x1
  virtio_mmio: virtio_mmio setting IRQ 0

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