[PATCH v2] arch/powerpc/pseries: Fix KVM guest detection for disabling hardlockup detector

[PATCH v2] arch/powerpc/pseries: Fix KVM guest detection for disabling hardlockup detector

[Gautam Menghani]

As per the kernel documentation[1], hardlockup detector should be
disabled in KVM guests as it may give false positives. On PPC, hardlockup
detector is enabled inside KVM guests because disable_hardlockup_detector()
is marked as early_initcall and it relies on kvm_guest static key 
(is_kvm_guest()) which is initialized later during boot by 
check_kvm_guest(), which is a core_initcall. check_kvm_guest() is also 
called in pSeries_smp_probe(), which is called before initcalls, but it is
skipped if KVM guest does not have doorbell support or if the guest is 
launched with SMT=1.

Call check_kvm_guest() in disable_hardlockup_detector() so that
is_kvm_guest() check goes through fine and hardlockup detector can be
disabled inside the KVM guest.

[1]: Documentation/admin-guide/sysctl/kernel.rst

Fixes: 633c8e9800f3 ("powerpc/pseries: Enable hardlockup watchdog for PowerVM partitions")
Cc: stable@vger.kernel.org # v5.14+
Signed-off-by: Gautam Menghani <gautam@linux.ibm.com>
---
v1 -> v2:
1. Call check_kvm_guest() call in disable_hardlockup_detector() instead
of relying on pSeries_smp_probe()
2. Add the fixes and stable tags
3. Some changes in patch description

 arch/powerpc/kernel/setup_64.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
index 22f83fbbc762..1edc7cd68c10 100644
--- a/arch/powerpc/kernel/setup_64.c
+++ b/arch/powerpc/kernel/setup_64.c
@@ -920,6 +920,7 @@ static int __init disable_hardlockup_detector(void)
 	hardlockup_detector_disable();
 #else
 	if (firmware_has_feature(FW_FEATURE_LPAR)) {
+		check_kvm_guest();
 		if (is_kvm_guest())
 			hardlockup_detector_disable();
 	}
-- 
2.47.0

Re: [PATCH v2] arch/powerpc/pseries: Fix KVM guest detection for disabling hardlockup detector

[Michael Ellerman]

On Fri, 08 Nov 2024 15:18:37 +0530, Gautam Menghani wrote:
> As per the kernel documentation[1], hardlockup detector should be
> disabled in KVM guests as it may give false positives. On PPC, hardlockup
> detector is enabled inside KVM guests because disable_hardlockup_detector()
> is marked as early_initcall and it relies on kvm_guest static key
> (is_kvm_guest()) which is initialized later during boot by
> check_kvm_guest(), which is a core_initcall. check_kvm_guest() is also
> called in pSeries_smp_probe(), which is called before initcalls, but it is
> skipped if KVM guest does not have doorbell support or if the guest is
> launched with SMT=1.
> 
> [...]

Applied to powerpc/next.

[1/1] arch/powerpc/pseries: Fix KVM guest detection for disabling hardlockup detector
      https://git.kernel.org/powerpc/c/44e5d21e6d3fd2a1fed7f0327cf72e99397e2eaf

cheers