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Re: [Qemu-discuss] [Qemu-devel] Cross-posted : Odd QXL/KVM performance i

From: Brad Campbell
Subject: Re: [Qemu-discuss] [Qemu-devel] Cross-posted : Odd QXL/KVM performance issue with a Windows 7 Guest
Date: Sat, 7 Sep 2019 08:44:35 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

On 7/9/19 03:03, Dr. David Alan Gilbert wrote:
* Brad Campbell (address@hidden) wrote:
On 2/9/19 6:23 pm, Brad Campbell wrote:

Here is the holdup :

11725@1567416625.003504:qxl_ring_command_check 0 native
11725@1567416625.102653:qxl_io_write 0 native addr=0 (QXL_IO_NOTIFY_CMD)
val=0 size=1 async=0

~100ms delay prior to each logged QXL_IO_NOTIFY_CMD on the AMD box which
explains the performance difference. Now I just need to figure out if
that lies in the guest, the guest QXL driver, QEMU or SPICE and why it
exhibits on the AMD box and not the i7.

To get to this point, I recompiled the kernel on the i7 box with both
AMD and Intel KVM modules. Once that was running I cloned the drive and
put it in the AMD box, so the OS, software stack and all dependencies
are identical.
Reacp :

I have a machine with a Windows 7 VM which is running on an i7-3770. This
works perfectly.

Clone the disk and put it in a new(ish) AMD Ryzen 1500x machine and the
display output using qxl/spice is now limited to ~5-7fps.

I originally cloned the entire machine to keep the software versions

To simplify debugging and reproduction I'm now using :
- An identical SPICE version to that on the i7.
- A fresh 64 bit Windows 7 VM.
- The D2D benchmark from Crystalmark 2004R7.

The machine is booted with :

qemu -enable-kvm \
  -m 8192\
  -rtc base=localtime\
  -vga qxl\
  -device qxl\
  -global qxl-vga.guestdebug=3\
  -global qxl-vga.cmdlog=1\
  -global qxl-vga.vram_size=65536\
  -global qxl.vram_size=65536\
  -global qxl-vga.ram_size=65536\
  -global qxl.ram_size=65536\
  -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 3,maxcpus=3,cores=3,threads=1,sockets=1\
  -cpu qemu64,hv_relaxed,hv_spinlocks=0x1fff,hv_vapic,hv_time \
-cpu qemu64 is almost always a bad idea;  does -cpu host help ?


No. I was using -cpu host. I changed it to qemu64 for testing so I could add & 
remove -enable-kvm for testing without the machine changing drivers about.



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

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