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    Subject[PATCH 5.13 123/380] powerpc/smp: Fix a crash while booting kvm guest with nr_cpus=2
    Date
    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


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