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>

2 files changed, 17 insertions, 1 deletions

diff --git a/arch/powerpc/kernel/paca.c b/arch/powerpc/kernel/paca.c
index bf0aada02fe4..ad302f845e5d 100644
--- a/arch/powerpc/kernel/paca.c
+++ b/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;
diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
index d8aabbdc6483..1d33e817ab2d 100644
--- a/arch/powerpc/kernel/setup_64.c
+++ b/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 */