[PATCH v2 1/3] powerpc/smp: Fix a crash while booting kvm guest with nr_cpus=2

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

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.

Cc: linuxppc-dev@lists.ozlabs.org
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Nathan Lynch <nathanl@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: Gautham R Shenoy <ego@linux.vnet.ibm.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Fixes: c1e53367dab1 ("powerpc/smp: Cache CPU to chip lookup")
Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Debugged-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
---
Changelog: v1 -> v2:
v1: - https://lore.kernel.org/linuxppc-dev/20210821092419.167454-2-srikar@linux.vnet.ibm.com/t/#u
Handled comment from Gautham Shenoy
[ Updated to use DIV_ROUND_UP instead of max to handle more situations ]

 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 6c6e4d934d86..bf11b3c4eb28 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1074,7 +1074,7 @@ 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,
-- 
2.18.2