The Lowest Priority Bug Ever? (/etc/udev/rules.d/z10-xen-vcpu-hotplug.rules)

[bnerickson]

I'm unsure where to report this, but syslog reports the following warning when a VM starts:

(UDEV-WORKER)   cpu0: Process '/bin/sh -c '[ -e /dev/xen/xenbus ] && [ -e /sys/devices/system/cpu/cpu0/online ] && echo 1 > /sys/devices/system/cpu/cpu0/online'' failed with exit code 1.

This is because /sys/devices/system/cpu/cpu0/online does not exist. I'm assuming that this may be expected, so I think the udev rule could be rewritten as follows to avoid spitting out an error:

# cat /etc/udev/rules.d/z10-xen-vcpu-hotplug.rules
ACTION=="add", SUBSYSTEM=="cpu", RUN+="/bin/sh -c 'if [ -e /dev/xen/xenbus ] && [ -e /sys$devpath/online ]; then echo 1 > /sys$devpath/online; fi'"

[bnerickson]

Oh, syslog also reports the following harmless warning:

Configuration file /etc/udev/rules.d/z10-xen-vcpu-hotplug.rules is marked executable. Please remove executable permission bits. Proceeding anyway.  4

So presumably the executable bits should be removed from this file as well.

For what it's worth:

dom0 version:

xcp-ng-release-config-8.3.0-35.x86_64
xcp-ng-deps-8.3-13.noarch
xcp-ng-plymouth-theme-1.1.0-5.xcpng8.3.noarch
xcp-ng-xapi-plugins-1.15.0-1.xcpng8.3.noarch
xcp-ng-generic-lib-1.1.1-4.xcpng8.3.x86_64
xcp-ng-release-presets-8.3.0-35.x86_64
xcp-ng-release-8.3.0-35.x86_64
xcp-ng-pv-tools-8.3-15.xcpng8.3.noarch

VM Details:
Kernel:

$ uname -a
Linux fqdn 6.12.0-124.27.1.el10_1.x86_64 #1 SMP PREEMPT_DYNAMIC Thu Jan 15 13:51:25 UTC 2026 x86_64 GNU/Linux

OS:

$ cat /etc/os-release 
NAME="Rocky Linux"
VERSION="10.1 (Red Quartz)"
ID="rocky"
ID_LIKE="rhel centos fedora"
VERSION_ID="10.1"
PLATFORM_ID="platform:el10"
PRETTY_NAME="Rocky Linux 10.1 (Red Quartz)"
ANSI_COLOR="0;32"
LOGO="fedora-logo-icon"
CPE_NAME="cpe:/o:rocky:rocky:10::baseos"
HOME_URL="https://rockylinux.org/"
VENDOR_NAME="RESF"
VENDOR_URL="https://resf.org/"
BUG_REPORT_URL="https://bugs.rockylinux.org/"
SUPPORT_END="2035-05-31"
ROCKY_SUPPORT_PRODUCT="Rocky-Linux-10"
ROCKY_SUPPORT_PRODUCT_VERSION="10.1"
REDHAT_SUPPORT_PRODUCT="Rocky Linux"
REDHAT_SUPPORT_PRODUCT_VERSION="10.1"

[olivierlambert]

Worth asking @Team-OS-Platform-Release

[bnerickson]

@Team-OS-Platform-Release , lowest priority ever, but any thoughts on the topic at hand?

[rzr]

@bnerickson said:

Thanks for the reminder, for some reason it was not seen before (all applogies).

I'm unsure where to report this,

Here or in related open projects directly (assuming you know where to look at) but let me guide you.

but syslog reports the following warning when a VM starts:

(UDEV-WORKER)   cpu0: Process '/bin/sh -c '[ -e /dev/xen/xenbus ] && [ -e /sys/devices/system/cpu/cpu0/online ] && echo 1 > /sys/devices/system/cpu/cpu0/online'' failed with exit code 1.

Good catch, it's a bad habit to use AND (&&) in conditionals.

Hardcore programmers prefer to use OR (||) in scripts, this practice prevents failed exit and make debugging easier (with set -xe).

(...)

> ACTION=="add", SUBSYSTEM=="cpu", RUN+="/bin/sh -c 'if [ -e /dev/xen/xenbus ] && [ -e /sys$devpath/online ]; then echo 1 > /sys$devpath/online; fi'"

That would work, What about this "simpler" form ?

[ ! -e /dev/xen/xenbus ]  || [ ! -e /sys$devpath/online ] || echo 1 > /sys$devpath/online 

Could be harder to read, but I get used to.

If you want you can contribute the fix directly to upstream since the file is public at:

https://github.com/xenserver/xe-guest-utilities/blob/master/mk/xen-vcpu-hotplug.rules

About the exec bit, here is the line that should be changed:

https://github.com/xcp-ng-rpms/xcp-ng-pv-tools/commit/3d134fbd0f3ac9e2f3cfa914e38b33529635d458#r183816712

@bnerickson, If you need any mentoring for OSS contributions, I would be more than a happy to help.
Since it's a lowest priority you can take all the time you need, and If too busy, no problem we can do it for you and then backport the change to XCP-ng.

1 stormi committed to xcp-ng-rpms/xcp-ng-pv-tools

Build the tools from the sources

- Build 32 bit and 64 bit xe-guest-utilities
- Build TGZ, RPM and DEB packages for xe-guest-utilities
- Put the result into the ISO and the ISO into the xcp-ng-pv-tools RPM
  (as before).

It's a bit hacky, but as the binaries are statically linked, this
peculiar build process should work.