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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.13 123/380] powerpc/smp: Fix a crash while booting kvm guest with nr_cpus=2
Date:   Thu, 16 Sep 2021 17:58:00 +0200
Message-Id: <20210916155808.229133033@linuxfoundation.org>
In-Reply-To: <20210916155803.966362085@linuxfoundation.org>
References: <20210916155803.966362085@linuxfoundation.org>
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From: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

[ Upstream commit 8efd249babea2fec268cff90b9f5ca723dbb7499 ]

Aneesh reported a crash with a fairly recent upstream kernel when
booting kernel whose commandline was appended with nr_cpus=2

1:mon> e
cpu 0x1: Vector: 300 (Data Access) at [c000000008a67bd0]
    pc: c00000000002557c: cpu_to_chip_id+0x3c/0x100
    lr: c000000000058380: start_secondary+0x460/0xb00
    sp: c000000008a67e70
   msr: 8000000000001033
   dar: 10
 dsisr: 80000
  current = 0xc00000000891bb00
  paca    = 0xc0000018ff981f80   irqmask: 0x03   irq_happened: 0x01
    pid   = 0, comm = swapper/1
Linux version 5.13.0-rc3-15704-ga050a6d2b7e8 (kvaneesh@ltc-boston8) (gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0, GNU ld (GNU Binutils for Ubuntu) 2.34) #433 SMP Tue May 25 02:38:49 CDT 2021
1:mon> t
[link register   ] c000000000058380 start_secondary+0x460/0xb00
[c000000008a67e70] c000000008a67eb0 (unreliable)
[c000000008a67eb0] c0000000000589d4 start_secondary+0xab4/0xb00
[c000000008a67f90] c00000000000c654 start_secondary_prolog+0x10/0x14

Current code assumes that num_possible_cpus() is always greater than
threads_per_core. However this may not be true when using nr_cpus=2 or
similar options. Handle the case where num_possible_cpus() is not an
exact multiple of  threads_per_core.

Fixes: c1e53367dab1 ("powerpc/smp: Cache CPU to chip lookup")
Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Debugged-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210826100401.412519-2-srikar@linux.vnet.ibm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/powerpc/kernel/smp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index df6b468976d5..7da1e01e2c7f 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1085,7 +1085,7 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
 	}
 
 	if (cpu_to_chip_id(boot_cpuid) != -1) {
-		int idx = num_possible_cpus() / threads_per_core;
+		int idx = DIV_ROUND_UP(num_possible_cpus(), threads_per_core);
 
 		/*
 		 * All threads of a core will all belong to the same core,
-- 
2.30.2