[Qemu-devel] [Bug 1253563] [NEW] bad performance with rng-egd backend

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You have been subscribed to a public bug by Amos Kong (amoskong):


1. create listen socket
# cat /dev/random | nc -l localhost 1024

2. start vm with rng-egd backend

./x86_64-softmmu/qemu-system-x86_64 --enable-kvm -mon 
chardev=qmp,mode=control,pretty=on -chardev 
socket,id=qmp,host=localhost,port=1234,server,nowait -m 2000 -device 
virtio-net-pci,netdev=h1,id=vnet0 -netdev tap,id=h1 -vnc :0 -drive 
file=/images/RHEL-64-virtio.qcow2 \
-chardev socket,host=localhost,port=1024,id=chr0 \
-object rng-egd,chardev=chr0,id=rng0 \
-device virtio-rng-pci,rng=rng0,max-bytes=1024000,period=1000

(guest) # dd if=/dev/hwrng of=/dev/null

note: cancelling dd process by Ctrl+c, it will return the read speed.

Problem:   the speed is around 1k/s

===================

If I use rng-random backend (filename=/dev/random), the speed is about
350k/s).

It seems that when the request entry is added to the list, we don't read the 
data from queue list immediately.
The chr_read() is delayed, the virtio_notify() is delayed.  the next request 
will also be delayed. It effects the speed.

I tried to change rng_egd_chr_can_read() always returns 1,  the speed is
improved to (about 400k/s)

Problem: we can't poll the content in time currently


Any thoughts?

Thanks, Amos

** Affects: qemu
     Importance: Undecided
         Status: New

-- 
bad performance with rng-egd backend
https://bugs.launchpad.net/bugs/1253563
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