[Bug 225506] Reading from virtio-blk device hangs

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=225506

            Bug ID: 225506
           Summary: Reading from virtio-blk device hangs
           Product: Base System
           Version: 11.1-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: kozlov.sergey.404 at gmail.com

I'm using FreeBSD on KVM virtual machine from a hosting provider.

On both 11.1-RELEASE and 12.0-CURRENT (2018-01-18) reading /dev/vtbd0 device
hangs. 
Using CentOS 6.9 I'm able to read full /dev/vda device without a hang.

- Command
  # dd if=/dev/vtbd0 of=/dev/null bs=1M
  will eventually hang.

- top shows dd in physrd state.
- gstat shows L(q) equal to 1 or 2 (depending on a try), all the IOPS and
throughput indications are zero for /dev/vtbd0
- dd can't be terminated with SIGKILL
- the only way to terminate the dd process is
  # devctl disable vtbd0

Server configuration:
RAM: 512MB
CPU: 1x Intel Xeon E5-2620 v3 @ 2.40 GHz
Disk: 250GB virtio-blk
<Additional info will be attached in a separate files>

The info from the hosting provider:
We are running the latest QEMU version and the latest 3.1 branch kernel, which
was released for fixing the Spectre and Meltdown vulnerabilities.

I'm still paying for the server, but I want to leave it in this state for now
to help investigate the issue.

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