machine/qemu*: fix kernel finish crng init more and more slowly

Just adding `-device virtio-rng-pci' to the QEMU invocation will
add the device with a default host backend. As of QEMU 1.3+,
the default backend is to use the host's /dev/random as a
source of entropy. [1]

When the entropy pool is empty, reads from /dev/random will
block until additional environmental noise is gathered. [2]

For Yocto, if call runqemu frequently, it will consume lots
of host's /dev/random, and kernel finish crng init in guest get
more and more slowly.

Here are 4 times runqemu boot test:

[    3.464432] random: crng init done
[   20.874030] random: crng init done
[   23.583589] random: crng init done
[   23.858945] random: crng init done

Modify entropy source to /dev/urandom device on the host which
returns random bytes using a pseudorandom number generator seeded
from the entropy pool. Reads from this device do not block and
kernel finish crng init in guest will not delay.

Of course, the side effect is obviously, we lost the quality of
randomness, but the modification is only on runqemu script
rather than real embedded device, and it benefits oeqa efficiency
in which many cases call runqemu especially multiple oeqa builds
on one host.

After apply the fix:

[    3.364670] random: crng init done
[    4.619061] random: crng init done
[    3.403897] random: crng init done
[    3.450717] random: crng init done

[1] https://wiki.qemu.org/Features/VirtIORNG
[2] http://man7.org/linux/man-pages/man4/random.4.html

(From OE-Core rev: 853644f82eb3205ef3efc1ea3959c7225dfacf61)

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/meta/conf/machine/qemuarm.conf b/meta/conf/machine/qemuarm.conf
index d2f2c85c3f..a544312627 100644
--- a/meta/conf/machine/qemuarm.conf
+++ b/meta/conf/machine/qemuarm.conf
@@ -14,7 +14,8 @@ SERIAL_CONSOLES ?= "115200;ttyAMA0 115200;ttyAMA1"
 QB_SYSTEM_NAME = "qemu-system-arm"
 QB_MACHINE = "-machine versatilepb"
 QB_KERNEL_CMDLINE_APPEND = "console=ttyAMA0,115200 console=tty"
+QB_OPT_APPEND = "-show-cursor -usb -device usb-tablet"
 # Add the 'virtio-rng-pci' device otherwise the guest may run out of entropy
-QB_OPT_APPEND = "-show-cursor -usb -device usb-tablet -device virtio-rng-pci"
+QB_OPT_APPEND += "-object rng-random,filename=/dev/urandom,id=rng0 -device virtio-rng-pci,rng=rng0"
 PREFERRED_VERSION_linux-yocto ??= "4.18%"
 QB_DTB = "${@oe.utils.version_less_or_equal('PREFERRED_VERSION_linux-yocto', '4.7', '', 'zImage-versatile-pb.dtb', d)}"