Xen HVM domU VNC not refreshing screen - linux

Question

On one of our hypervisors running Xen (v.4.6.0 on top of Debian Jessie on a Dell R420), when we configure a domU for HVM and connect to the console via VNC, the connection displays a static image and appears to not accept mouse or keyboard input (leading you to think that the VM is frozen/not responsive). The behavior persists after closing and reconnecting over VNC, but the mouse/keyboard input from the previous session is now reflected (so if you tab three times, you can see that the appropriate radio or input button is highlighted after closing/opening the VNC connection, but you need to close the window again to see where the next input is, making it unusable).
We have Xen running smoothly on three other physical machines with HVM-configured domUs (2x Debian Jessie, 1x Ubuntu Xenial, all with v.4.6.0) and have been comparing what could be different, we noticed that QEMU could be updated on the troublesome Xen host. After upgrading QEMU from 1.2.2 to 1.2.5 (matching the version on the working hosts) and rebooting, the issue still persists. We have copied the VM config to another host with successful results, leading us to believe there is something isolated to this machine.
Results of cat /sys/hypervisor/properties/capabilities
xen-3.0-x86_64 xen-3.0-x86_32p hvm-3.0-x86_32 hvm-3.0-x86_32p hvm-3.0-x86_64
Results of xl info:
host : vm-host
release : 3.16.0-4-amd64
version : #1 SMP Debian 3.16.7-ckt25-2+deb8u3 (2016-07-02)
machine : x86_64
nr_cpus : 16
max_cpu_id : 47
nr_nodes : 1
cores_per_socket : 8
threads_per_core : 2
cpu_mhz : 2500
hw_caps : bfebfbff:2c100800:00000000:00007f00:77bee3ff:00000000:00000001:00000281
virt_caps : hvm hvm_directio
total_memory : 32704
free_memory : 17945
sharing_freed_memory : 0
sharing_used_memory : 0
outstanding_claims : 0
free_cpus : 0
xen_major : 4
xen_minor : 6
xen_extra : .0
xen_version : 4.6.0
xen_caps : xen-3.0-x86_64 xen-3.0-x86_32p hvm-3.0-x86_32 hvm-3.0-x86_32p hvm-3.0-x86_64
xen_scheduler : credit
xen_pagesize : 4096
platform_params : virt_start=0xffff800000000000
xen_changeset :
xen_commandline : placeholder dom0_mem=1024M,max:1024M dom0_max_vcpus=1 dom0_vcpus_pin no-real-mode edd=off
cc_compiler : gcc (Debian 5.3.1-8) 5.3.1 20160205
cc_compile_by : ijc
cc_compile_domain : debian.org
cc_compile_date : Tue Feb 9 17:46:27 UTC 2016
xend_config_format : 4
Sample domU config:
name="VM1"
uuid="91f4c306-101b-431b-bf73-2146b2a137fb"
vcpus=2
memory=2048
disk = [ "phy:/dev/vg1/centos,xvda2,w",
"file:/path/folder/images/CentOS-7-x86_64-Minimal-511.iso,xvdb:cdrom,r" ]
builder = "hvm"
boot = "dc"
vnc = "1"
vnclisten = "0.0.0.0"
vncdisplay = "0"
vncpasswd = "password"
vga ="stdvga"
videoram = 64
Any and all advice on how to get VNC working smoothly and properly would be greatly appreciated!

Answer 1

Try add GRUB_GFXPAYLOAD_LINUX="keep" or GRUB_GFXPAYLOAD_LINUX="640x480" (or another resolution) into /etc/default/grub on DomU and then run update-grub2 (on DomU) and reboot. This helped me with the same error.

Answer 2

Thanks for the recommendation. It turned out that we had mixed versions of Xen and its dependencies installed (some 4.4, some 4.6). We ended up removing Xen and all related packages and reinstalling. During installation, we noticed that installing xen-hypervisor-4.6-amd64 was coming from the stretch repo (expected), but its dependencies were coming from the jessie main repo with older versions (e.g., libxen-4.4 instead of libxen-4.6). To solve it, we ran
apt-get -t stretch install xen-hypervisor-4.6-amd64
which properly installed all dependencies from stretch, and after a reboot, VNC connections to HVM domU were working as expected.