powerpc/ppc64: Gracefully handle early interrupts

If we take an interrupt such as a trap caused by a BUG_ON before the
MMU has been setup, the interrupt handlers try to enable virutal mode
and cause a recursive crash, making the original problem very hard
to debug.

This fixes it by adjusting the "kernel_msr" value in the PACA so that
it only has MSR_IR and MSR_DR (translation for instruction and data)
set after the MMU has been initialized for the processor.

We may still not have a console yet but at least we don't get into
a recursive fault (and early debug console or memory dump via JTAG
of the kernel buffer *will* give us the proper error).
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/paca.c

@@ -152,7 +152,8 @@ void __init initialise_paca(struct paca_struct *new_paca, int cpu)
 	new_paca->paca_index = cpu;
 	new_paca->kernel_toc = kernel_toc;
 	new_paca->kernelbase = (unsigned long) _stext;
-	new_paca->kernel_msr = MSR_KERNEL;
+	/* Only set MSR:IR/DR when MMU is initialized */
+	new_paca->kernel_msr = MSR_KERNEL & ~(MSR_IR | MSR_DR);
 	new_paca->hw_cpu_id = 0xffff;
 	new_paca->kexec_state = KEXEC_STATE_NONE;
 	new_paca->__current = &init_task;

arch/powerpc/kernel/setup_64.c

@@ -261,6 +261,14 @@ void __init early_setup(unsigned long dt_ptr)
 	/* Initialize the hash table or TLB handling */
 	early_init_mmu();
 
+	/*
+	 * At this point, we can let interrupts switch to virtual mode
+	 * (the MMU has been setup), so adjust the MSR in the PACA to
+	 * have IR and DR set.
+	 */
+	get_paca()->kernel_msr = MSR_KERNEL;
+
+	/* Reserve large chunks of memory for use by CMA for KVM */
 	kvm_cma_reserve();
 
 	/*
@@ -293,6 +301,13 @@ void early_setup_secondary(void)
 
 	/* Initialize the hash table or TLB handling */
 	early_init_mmu_secondary();
+
+	/*
+	 * At this point, we can let interrupts switch to virtual mode
+	 * (the MMU has been setup), so adjust the MSR in the PACA to
+	 * have IR and DR set.
+	 */
+	get_paca()->kernel_msr = MSR_KERNEL;
 }
 
 #endif /* CONFIG_SMP */