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

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Message-ID: <87ed3ncl8q.fsf@mpe.ellerman.id.au>
Date: Thu, 07 Nov 2024 22:54:29 +1100
From: Michael Ellerman <mpe@...erman.id.au>
To: Gautam Menghani <gautam@...ux.ibm.com>, 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: Re: [PATCH] arch/powerpc/pseries: Fix KVM guest detection for
 disabling hardlockup detector

Gautam Menghani <gautam@...ux.ibm.com> writes:
> 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()
 
Isn't it the opposite? Inside KVM guests, the hardlockup detector should
be *disabled*, but it's not it's *enabled*, due to this bug.

ie. it's not broken, it's working, but that's the bug.

> 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.

I'm wondering how no one has noticed. Most KVM guests have 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.

check_kvm_guest() is safe to be called multiple times so
disable_hardlockup_detector() should just call it before it calls
is_kvm_guest(). That should avoid future breakage when the order of
calls changes, or someone refactors pSeries_smp_probe().

Can you identify the commit that broke this and include a Fixes: tag
please.

cheers

> [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