On 5900x Zen3 system, ubuntu 26.04 LTS , kernel 7.0.0-38-generic :

Test A1) without xen

To "GRUB_CMDLINE_LINUX_DEFAULT" added "maxcpus=12"

* boots normally ...
* htop lists all thread but half are [offline]
* nproc result: 12

To "GRUB_CMDLINE_LINUX_DEFAULT" added "maxcpus=12 nvme.poll_queues=24"

Same result, interesting, so "maxcpus" parameter not related to this
bug.


Test A2) with xen

Same system, Installed xen-hypervisor-4.20-amd64

"GRUB_CMDLINE_XEN_DEFAULT" undefined ""
"GRUB_CMDLINE_LINUX_DEFAULT" is cleared ""
GRUB booting to Xen

* stuck on black screen.. waiting.. 8 min nothing..
numlock light toggles on keypress
ctrl alt F1/F2/F3 - not working
ctrl+alt+del - does work

Well this is unexpected.

Aha there is one difference here.
SSH does works, system is actually up.
# xl info | grep xen_v
xen_version            : 4.20.3-pre

System has Cinnamon Desktop so I suspect it is UI related.
Not same issue as Supermicro headless system had where SSH was also 
inaccessible.


Ok I will use SSH just to finish testing what we need here

Rebooting with:
"GRUB_CMDLINE_XEN_DEFAULT" undefined "dom0_max_vcpus=12"
"GRUB_CMDLINE_LINUX_DEFAULT" is cleared ""

* boots to black screen, SSH logs in after 30 sec 
(30s seems to be normal Xen boot timing)

Now this is completely different
* htop no [offline] threads, only 0 to 11 so ttoal 12 threads lsited.
* nproc result: 12


Rebooting with:
"GRUB_CMDLINE_XEN_DEFAULT" undefined "dom0_max_vcpus=12"
"GRUB_CMDLINE_LINUX_DEFAULT" is cleared "nvme.poll_queues=24"
update-grub , reboot

* boots to black screen, waiting , ok SSH has been retrying for 6min -
nothing.

So here we have it - reproducible on any Zen3 system.

Probably on any AMD and Intel system I suspect.

PS:

If SSH does not come up, quite sure there is no networking for
netconsole.

This MB has no serial port (might have internal, but I do not have the
wires/adapters to do the connection), sorry.

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https://bugs.launchpad.net/bugs/2170270

Title:
  Xen Dom0 fails to boot with "nvme.poll_queues=24"  and
  "dom0_max_vcpus=6 dom0_vcpus_pin"

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