Disable all watchdogs and blacklist their kernel modules

We don't need a watchdog for normal desktop use cases. Expressing that
intention, we had already set kernel.nmi_watchdog = 0 to disable the
kernel panic watchdog.

We can go further and instead use the "nowatchdog" kernel parameter to
disable *all* of them, and then additionally blacklist their kernel
modules. This should have a very slightly positive impact on battery
life and boot/shutdown times.

mkosi.extra/usr/lib/modprobe.d/98-kde-linux-watchdog.conf

@@ -0,0 +1,15 @@
+# SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+# SPDX-FileCopyrightText: none
+
+# We disable watchdogs by default in KDE Linux using the "nowatchdog" kernel parameter,
+# as they aren't relevant to normal desktop use cases. So also disable their unused
+# kernel modules to reduce power usage and shave milliseconds off boot/shutdown times:
+
+# Intel
+blacklist iTCO_wdt
+
+# AMD
+blacklist sp5100_tco
+
+# Miscellaneous
+blacklist wdat_wdt

mkosi.extra/usr/lib/rebuild-efi

@@ -34,6 +34,7 @@ echo "rw \
   nohz=on nohz_full=all \
   rcu_nocbs=all rcutree.enable_rcu_lazy=1 \
   amdgpu.dcdebugmask=0x10 \
+  nowatchdog \
   vt.global_cursor_default=0 quiet splash plymouth.ignore-serial-consoles loglevel=3" > cmdline
 mkinitcpio --config mkinitcpio.conf --generate initrd --kernel "$kernel_version"
 ukify build \
@@ -55,6 +56,8 @@ ukify build \
 # automatically by nohz_full=all, but we manually specify it here anyway because
 # rcutree.enable_rcu_lazy=1 needs it and we don't want that to break.
 
+# "nowatchdog" turns off all watchdog functionality.
+
 # amdgpu.dcdebugmask=0x10 works around https://gitlab.freedesktop.org/drm/amd/-/issues/4831
 echo "rw rootflags=subvol=@system,compress=zstd:-4,lazytime \
   lsm=landlock,lockdown,yama,integrity,bpf \
@@ -64,6 +67,7 @@ echo "rw rootflags=subvol=@system,compress=zstd:-4,lazytime \
   rcu_nocbs=all rcutree.enable_rcu_lazy=1 \
   amdgpu.dcdebugmask=0x10 \
   systemd.hostname=kde-linux \
+  nowatchdog \
   vt.global_cursor_default=0 quiet splash plymouth.ignore-serial-consoles loglevel=3" > cmdline
 mkinitcpio --config mkinitcpio.conf --generate initrd --kernel "$kernel_version"
 ukify build \

mkosi.extra/usr/lib/sysctl.d/11-kde-linux-default.conf

@@ -15,8 +15,6 @@ vm.watermark_boost_factor = 0
 vm.watermark_scale_factor = 125
 # Many Windows games need this disabled to run with proper performance; they abuse split locks
 kernel.split_lock_mitigate = 0
-# Not needed for us; disabling it increases speed and reduces power consumption
-kernel.nmi_watchdog = 0
 # Use 'bbr' to achieve higher throughput when sending to high-latency destinations.
 # Also 'fq' to prevent one greedy app from causing lag (bufferbloat) for everything else.
 # `bbr` relies on pacing, and thus performs better with the `fq` qdisc.