[Qemu-discuss] Odd QXL performance issue with a Windows 7 Guest

[Brad Campbell]

G'day all,

I have a box which has the sole purpose of running a Windows 7 VM. It's 
an i7-3770K built in 2013 and happily runs a Windows 7 VM with 2 QXL 
heads which are displayed remotely over a 1GB ethernet to a client 
running the spicy spice-gtk client.

This works extremely well, and has been pretty flawless over the years 
I've used it.

A year or so ago I built a pair of AMD Ryzen boxes and I thought I might 
be able to migrate the VM across to one of the Ryzen boxes and 
decommission the i7. No issues doing that *except* in the display 
performance. This has made the VM unusable for the task and I've been 
trying to figure out what is going on. That led to be running 
"Crystalmark" to "benchmark" the relative machines and was most 
illuminating.

CPU/FPU/Memory/Storage are all comparable. GDI performance is similar, 
but when we get to the D2D test the Intel box runs at about ~50 FPS and 
the AMD box runs ~5FPS. That explains the interactive issues when 
running CAD in the guest.

So, to try and debug this I've tried :
- New (current) Spice/QEMU build on the AMD box.
- Complete rsync of QEMU/Spice and all dependencies from the Intel to 
the AMD box.
- Clean Windows 7 guest install on the AMD box with the latest QEMU and 
guest Virtio drivers.
- Clean Windows 7 guest install on the AMD box with QEMU/Spice/Deps 
rsynced across from the Intel box.

I'm absolutely perplexed. I've dropped the guest down to a single head 
and removed the qxl driver reverting to the standard windows driver and 
that gives me > 100 FPS.

I've tried enabling simpletrace and run traces on both machines. Aside 
from the intel machine time deltas for the commands being ~ half those 
on the AMD box I really can spot anything strange.

Both are running recent kernels and aside from the AMD/Intel KVM 
selector are running the same KVM and timer configurations, so I'm a bit 
lost. Anyone have any ideas I can try on instrumenting any elements to 
try and find out what is going on?

I use this to launch the VM on both machines.

qemu -enable-kvm\
 -m 24576\
 -rtc base=localtime\
 -vga qxl\
 -device qxl\
 -net nic,model=virtio\
 -net tap,ifname=tap0,script=/etc/qemu-ifup,vhost=on\
 -usbdevice tablet\
 -spice port=5930,disable-ticketing\
 -device virtio-serial\
 -chardev spicevmc,id=vdagent,name=vdagent\
 -device virtserialport,chardev=vdagent,name=com.redhat.spice.0\
 -smp 4,maxcpus=4,cpus=4,cores=4,threads=1,sockets=1\
 -cpu kvm64,hv_relaxed,hv_spinlocks=0x1fff,hv_vapic,hv_time \
 -drive 
file=/server/VM/Cadbox.raw,if=virtio,aio=threads,format=raw,cache=unsafe 
-boot c \
 -drive 
file=/server/VM/Cadbox_swap.raw,if=virtio,aio=threads,format=raw,cache=unsafe 
\
 -monitor telnet:127.0.0.1:5002,server,nowait

Regards,
Brad
--
An expert is a person who has found out by his own painful
experience all the mistakes that one can make in a very
narrow field. - Niels Bohr