[PATCH] powerpc/64s/radix: Fix radix_kvm_prefetch_workaround paca access of not possible CPU

[Nicholas Piggin]

If possible CPUs are limited (e.g., by kexec), then the kvm prefetch
workaround function can access the paca pointer for a !possible CPU.

Fixes: d2e60075a3d44 ("powerpc/64: Use array of paca pointers and allocate 
pacas individually")
Cc: sta...@kernel.org
Reported-by: Pridhiviraj Paidipeddi <ppaid...@linux.vnet.ibm.com>
Tested-by: Pridhiviraj Paidipeddi <ppaid...@linux.vnet.ibm.com>
Signed-off-by: Nicholas Piggin <npig...@gmail.com>

---
[kdump still seems to have a problem upstream, but this solves one crash]

 arch/powerpc/mm/tlb-radix.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/powerpc/mm/tlb-radix.c b/arch/powerpc/mm/tlb-radix.c
index 67a6e86d3e7e..06ea845d8033 100644
--- a/arch/powerpc/mm/tlb-radix.c
+++ b/arch/powerpc/mm/tlb-radix.c
@@ -1043,6 +1043,8 @@ extern void radix_kvm_prefetch_workaround(struct 
mm_struct *mm)
                for (; sib <= cpu_last_thread_sibling(cpu) && !flush; sib++) {
                        if (sib == cpu)
                                continue;
+                       if (!cpu_possible(sib))
+                               continue;
                        if (paca_ptrs[sib]->kvm_hstate.kvm_vcpu)
                                flush = true;
                }
-- 
2.17.0