powerpc/64s: return more carefully from sreset NMI

System Reset, being an NMI, must return more carefully than other
interrupts. It has traditionally returned via the nromal return
from exception path, but that has a number of problems.

- r13 does not get restored if returning to kernel. This is for
  interrupts which may cause a context switch, which sreset will
  never do. Interrupting OPAL (which uses a different r13) is one
  place where this causes breakage.

- It may cause several other problems returning to kernel with
  preempt or TIF_EMULATE_STACK_STORE if it hits at the wrong time.

It's safer just to have a simple restore and return, like machine
check which is the other NMI.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index 1a0aa70bcb2ba8f85d41eeb4780a8fb5fd4595a9..bc5d927da37a8f36c1f24f5c89c3d12248c5d890 100644 (file)
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S
@@ -139,6 +139,21 @@ EXC_COMMON_BEGIN(system_reset_idle_common)
        b       pnv_powersave_wakeup
 #endif
 
+/*
+ * Set IRQS_ALL_DISABLED unconditionally so arch_irqs_disabled does
+ * the right thing. We do not want to reconcile because that goes
+ * through irq tracing which we don't want in NMI.
+ *
+ * Save PACAIRQHAPPENED because some code will do a hard disable
+ * (e.g., xmon). So we want to restore this back to where it was
+ * when we return. DAR is unused in the stack, so save it there.
+ */
+#define ADD_RECONCILE_NMI                                              \
+       li      r10,IRQS_ALL_DISABLED;                                  \
+       stb     r10,PACAIRQSOFTMASK(r13);                               \
+       lbz     r10,PACAIRQHAPPENED(r13);                               \
+       std     r10,_DAR(r1)
+
 EXC_COMMON_BEGIN(system_reset_common)
        /*
         * Increment paca->in_nmi then enable MSR_RI. SLB or MCE will be able
@@ -157,16 +172,56 @@ EXC_COMMON_BEGIN(system_reset_common)
        subi    r1,r1,INT_FRAME_SIZE
        EXCEPTION_COMMON_NORET_STACK(PACA_EXNMI, 0x100,
                        system_reset, system_reset_exception,
-                       ADD_NVGPRS;ADD_RECONCILE)
+                       ADD_NVGPRS;ADD_RECONCILE_NMI)
+
+       /* This (and MCE) can be simplified with mtmsrd L=1 */
+       /* Clear MSR_RI before setting SRR0 and SRR1. */
+       li      r0,MSR_RI
+       mfmsr   r9
+       andc    r9,r9,r0
+       mtmsrd  r9,1
 
        /*
-        * The stack is no longer in use, decrement in_nmi.
+        * MSR_RI is clear, now we can decrement paca->in_nmi.
         */
        lhz     r10,PACA_IN_NMI(r13)
        subi    r10,r10,1
        sth     r10,PACA_IN_NMI(r13)
 
-       b       ret_from_except
+       /*
+        * Restore soft mask settings.
+        */
+       ld      r10,_DAR(r1)
+       stb     r10,PACAIRQHAPPENED(r13)
+       ld      r10,SOFTE(r1)
+       stb     r10,PACAIRQSOFTMASK(r13)
+
+       /*
+        * Keep below code in synch with MACHINE_CHECK_HANDLER_WINDUP.
+        * Should share common bits...
+        */
+
+       /* Move original SRR0 and SRR1 into the respective regs */
+       ld      r9,_MSR(r1)
+       mtspr   SPRN_SRR1,r9
+       ld      r3,_NIP(r1)
+       mtspr   SPRN_SRR0,r3
+       ld      r9,_CTR(r1)
+       mtctr   r9
+       ld      r9,_XER(r1)
+       mtxer   r9
+       ld      r9,_LINK(r1)
+       mtlr    r9
+       REST_GPR(0, r1)
+       REST_8GPRS(2, r1)
+       REST_GPR(10, r1)
+       ld      r11,_CCR(r1)
+       mtcr    r11
+       REST_GPR(11, r1)
+       REST_2GPRS(12, r1)
+       /* restore original r1. */
+       ld      r1,GPR1(r1)
+       RFI_TO_USER_OR_KERNEL
 
 #ifdef CONFIG_PPC_PSERIES
 /*