powerpc/pseries: Limit machine check stack to 4GB

This allows rtas_args to be put on the machine check stack, which
avoids a lot of complications with re-entrancy deadlocks.

Signed-off-by: Nicholas Piggin <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Reviewed-by: Christophe Leroy <[e-mail]>
Reviewed-by: Mahesh Salgaonkar <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/powerpc/kernel/setup_64.c

···
  */
 void __init emergency_stack_init(void)
 {
-	u64 limit;
+	u64 limit, mce_limit;
 	unsigned int i;
 
 	/*
···
 	 * initialized in kernel/irq.c. These are initialized here in order
 	 * to have emergency stacks available as early as possible.
 	 */
-	limit = min(ppc64_bolted_size(), ppc64_rma_size);
+	limit = mce_limit = min(ppc64_bolted_size(), ppc64_rma_size);
+
+	/*
+	 * Machine check on pseries calls rtas, but can't use the static
+	 * rtas_args due to a machine check hitting while the lock is held.
+	 * rtas args have to be under 4GB, so the machine check stack is
+	 * limited to 4GB so args can be put on stack.
+	 */
+	if (firmware_has_feature(FW_FEATURE_LPAR) && mce_limit > SZ_4G)
+		mce_limit = SZ_4G;
 
 	for_each_possible_cpu(i) {
 		paca_ptrs[i]->emergency_sp = alloc_stack(limit, i) + THREAD_SIZE;
···
 		paca_ptrs[i]->nmi_emergency_sp = alloc_stack(limit, i) + THREAD_SIZE;
 
 		/* emergency stack for machine check exception handling. */
-		paca_ptrs[i]->mc_emergency_sp = alloc_stack(limit, i) + THREAD_SIZE;
+		paca_ptrs[i]->mc_emergency_sp = alloc_stack(mce_limit, i) + THREAD_SIZE;
 #endif
 	}
 }