FWIW, this bug seems related, and I also have "BUG: soft lockup detected on CPU#0!" in my domU's log, although I had not realized this was related to dmidecode. https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=201799
FWIW, one way I know to detect a xen domU is to check if /proc/xen/capabilities exists and does not contain the string "control_d". This is the method used by update-grub, and dmidecode could be modified to check for that and exit. I'm not sure if dmidecode works in a dom0, if it also fails there it could simply abort if /proc/xen/capabilities exists. -- see shy jo
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