KVM PR and KVM HV do not work if the kernel was compiled with PREEMPT

[chzigotzky]

KVM PR and KVM HV do not work if the kernel was compiled with PREEMPT.

We were previously able to disable PREEMPT in the kernel configuration, but the latest kernels now enable it by default and it is no longer possible to disable it.

[chzigotzky]

I created a patch for disabling PREEMPT today.

--- a/kernel/Kconfig.preempt	2026-02-14 10:06:32.000000000 +0000
+++ b/kernel/Kconfig.preempt	2026-02-08 21:03:27.000000000 +0000
@@ -16,13 +16,11 @@ config ARCH_HAS_PREEMPT_LAZY
 
 choice
 	prompt "Preemption Model"
-	default PREEMPT_LAZY if ARCH_HAS_PREEMPT_LAZY
 	default PREEMPT_NONE
 
 config PREEMPT_NONE
 	bool "No Forced Preemption (Server)"
 	depends on !PREEMPT_RT
-	depends on ARCH_NO_PREEMPT
 	select PREEMPT_NONE_BUILD if !PREEMPT_DYNAMIC
 	help
 	  This is the traditional Linux preemption model, geared towards
@@ -37,7 +35,6 @@ config PREEMPT_NONE
 
 config PREEMPT_VOLUNTARY
 	bool "Voluntary Kernel Preemption (Desktop)"
-	depends on !ARCH_HAS_PREEMPT_LAZY
 	depends on !ARCH_NO_PREEMPT
 	depends on !PREEMPT_RT
 	select PREEMPT_VOLUNTARY_BUILD if !PREEMPT_DYNAMIC

[chzigotzky]

Further information:

The entire FSL Cyrus+ board freezes when using KVM HV with PREEMPT.

The guest kernel doesn't boot if we use KVM PR with a PREEMPT kernel on
the PA Semi Nemo board.

I reported this issue to the kernel developers today.

Link: [Linux PPC] KVM PR and KVM HV do not work if the kernel was compiled with PREEMPT