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

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Message-ID: <20241105132734.499506-1-gautam@linux.ibm.com>
Date: Tue,  5 Nov 2024 18:57:33 +0530
From: Gautam Menghani <gautam@...ux.ibm.com>
To: mpe@...erman.id.au, npiggin@...il.com, christophe.leroy@...roup.eu,
        naveen@...nel.org, maddy@...ux.ibm.com
Cc: Gautam Menghani <gautam@...ux.ibm.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] arch/powerpc/pseries: Fix KVM guest detection for disabling hardlockup detector

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 broken inside KVM guests because disable_hardlockup_detector()
is marked as early_initcall and it uses is_kvm_guest(), which is
initialized by check_kvm_guest() later during boot as it 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.

Move the check_kvm_guest() call in pSeries_smp_probe() to the initial
part of function before doorbell/SMT checks so that "kvm_guest" static
key is initialized by the time disable_hardlockup_detector() runs.

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

Signed-off-by: Gautam Menghani <gautam@...ux.ibm.com>
---
 arch/powerpc/platforms/pseries/smp.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/smp.c b/arch/powerpc/platforms/pseries/smp.c
index c597711ef20a..516c7bfec933 100644
--- a/arch/powerpc/platforms/pseries/smp.c
+++ b/arch/powerpc/platforms/pseries/smp.c
@@ -199,6 +199,13 @@ static __init void pSeries_smp_probe(void)
 	else
 		xics_smp_probe();

+	/*
+	 * Make sure this is called regardless of doorbell/SMT status, as
+	 * we disable hardlockup detector in an early_initcall where we need to
+	 * know KVM status for disabling hardlockup detector in KVM guests.
+	 */
+	check_kvm_guest();
+
 	/* No doorbell facility, must use the interrupt controller for IPIs */
 	if (!cpu_has_feature(CPU_FTR_DBELL))
 		return;
@@ -207,8 +214,6 @@ static __init void pSeries_smp_probe(void)
 	if (!cpu_has_feature(CPU_FTR_SMT))
 		return;

-	check_kvm_guest();
-
 	if (is_kvm_guest()) {
 		/*
 		 * KVM emulates doorbells by disabling FSCR[MSGP] so msgsndp
-- 
2.47.0