[Qemu-discuss] Qemu on Windows (with haxm and whpx), minimal Ubuntu, SLIRP and OpenVPN: very slow

[Michael Fritscher]

Good day,

Scenario:
Host: Windows 7 and 10 64 bit, Skylake+ CPUs
Qemu Version: 3.1 (4.0 isn't available yet on https://qemu.weilnetz.de/)
Accelerators: HAXM and WHPX
Guest: Ubuntu 18.04.2 64 bit, minimal install
Guest's tasks: OpenVPN, iptables, tc

Configuration:
-machine q35,accel=kvm:hvf:whpx:hax:tcg
-m 512
-smp 1
-rtc base=utc
-drive file=layertc_environmentmanager_amsvm.iso,if=virtio,media=cdrom
-vga std
-netdev
        user,id=natted,
        restrict=n,
        net=172.31.1.0/255.255.255.0,
        dhcpstart=172.31.1.1,
        host=172.31.1.3,
        dns=172.31.1.4
-device virtio-net,netdev=natted,mac=00:50:56:00:08:00
-nodefaults
-monitor tcp:localhost:4444
-kernel vmlinuz
-initrd initrd.img
-append "boot=casper textonly quiet nosplash nofb fb=false pti=off spectre_v2=off 
l1tf=off nospec_store_bypass_disable noprompt"

(the iso is a casper livecd, but I start the linux kernel directly because of 
boot speed)

Problem:
It is slow...
While a openssl speed -elapsed -evp aes-128-cbc is ok (500MB/sec and beyond), 
OpenVPN struggles.
I get 70% virtual CPU usage , mostly on OpenVPN and system load while transfering 1-2 MB/s with a UDP video stream. The measured delay is also 
rather spiky and is about 30...40 ms. On the host I get a cpu usage of about 10% (=almost a core) I see this both on the server and the client side.

On the player edition of the big commercial player, the results are _much_ 
better (10% virtual CPU usage, delay of 4-6 ms)

How can I further debug this? Do you know any knobs I can try? Slirp, while there are warnings of being slow, doesn't seem to be the culprit as 
I can get easily >>10 MB/s through it. And I see the big CPU usage of OpenVPN

Best regards,
Michael Fritscher

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