Log files
On a XCP-ng host, like in most Linux/UNIX systems, the logs are located in /var/log. XCP-ng does not use journald for logs, so everything is in /var/log directly.
General log
/var/log/daemon.log
Output of various running daemons involved in XCP-ng's tasks. Examples: output of xenopsd which handles the communication with the VMs, of executables involved in live migration and storage motion, and more...
XAPI's log
/var/log/xensource.log
Contains the output of the XAPI toolstack.
Storage related (eg. coalescing snapshots)
/var/log/SMlog
Contains the output of the storage manager.
Kernel messages
For hardware related issues or system crashes.
/var/log/kern.log
All kernel logs since last boot: type dmesg.
Kernel crash logs
In case of a host crash, if it is kernel-related, you should find logs in /var/crash
Produce a status report
To help someone else identify an issue or reproduce a bug, you can generate a full status report containing all log files, details about your configuration and more.
xen-bugtool --yestoall
Then upload the resulting archive somewhere. It may contain sensitive information about your setup, so it may be better to upload it to a private area and give the link only to those you trust to analyze it.
XCP-ng Center
You can display the log files via menu Help -> View XCP-ng Center Log Files.
The log files are located in C:\Users\<user>\AppData\Roaming\XCP-ng\XCP-ng Center\logs.
Windows VM
(PV-)Driver install log
C:\Windows\INF\setupapi.dev.log
Useful data for debugging
DMAR/IVRS ACPI tables
To debug various issues (for example IOMMU-related issues), developers may need to consult the DMAR (Intel) or IVRS (AMD) acpi tables, extracted from the firmware.
Here's how to extract them, from a Linux system with acpica-tools (or equivalent name in your distro) installed, as root:
mkdir acpi && cd acpi
acpidump > acpi.dmp
acpixtract -a acpi.dmp
[[ -f rmad.dat ]] && echo "DMAR" | dd of=rmad.dat bs=1 count=4 conv=notrunc
iasl -d *.dat
This will produce either ivrs.dsl, rmad.dsl or dmar.dsl.