powerpc/smp: soft-replugged CPUs must go back to start_secondary

Various thing are torn down when a CPU is hot-unplugged. That CPU
is expected to go back to start_secondary when re-plugged to re
initialize everything, such as clock sources, maps, ...

Some implementations just return from cpu_die() callback
in the idle loop when the CPU is "re-plugged". This is not enough.

We fix it using a little asm trampoline which resets the stack
and calls back into start_secondary as if we were all fresh from
boot. The trampoline already existed on ppc64, but we add it for
ppc32

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/arch/powerpc/kernel/head_32.S b/arch/powerpc/kernel/head_32.S
index 98c4b29a56f4..c5c24beb8387 100644
--- a/arch/powerpc/kernel/head_32.S
+++ b/arch/powerpc/kernel/head_32.S
@@ -890,6 +890,15 @@ __secondary_start:
         mtspr   SPRN_SRR1,r4
         SYNC
         RFI
+
+_GLOBAL(start_secondary_resume)
+        /* Reset stack */
+        rlwinm  r1,r1,0,0,(31-THREAD_SHIFT)     /* current_thread_info() */
+        addi    r1,r1,THREAD_SIZE-STACK_FRAME_OVERHEAD
+        li      r3,0
+        std     r3,0(r1)                /* Zero the stack frame pointer */
+        bl      start_secondary
+        b       .
 #endif /* CONFIG_SMP */
 
 #ifdef CONFIG_KVM_BOOK3S_HANDLER