ppc: Fix hreg_store_msr() so that non-HV mode cannot alter MSR:HV

This helper is only used by the various instructions that can alter
MSR and not interrupts. Add a comment to that effect to the interrupt
code as well in case somebody wants to change this

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

target-ppc/excp_helper.c

@@ -709,8 +709,12 @@ static inline void powerpc_excp(PowerPCCPU *cpu, int excp_model, int excp)
         }
     }
 #endif
-    /* XXX: we don't use hreg_store_msr here as already have treated
-     *      any special case that could occur. Just store MSR and update hflags
+    /* We don't use hreg_store_msr here as already have treated
+     * any special case that could occur. Just store MSR and update hflags
+     *
+     * Note: We *MUST* not use hreg_store_msr() as-is anyway because it
+     * will prevent setting of the HV bit which some exceptions might need
+     * to do.
      */
     env->msr = new_msr & env->msr_mask;
     hreg_compute_hflags(env);

target-ppc/helper_regs.h

@@ -114,8 +114,8 @@ static inline int hreg_store_msr(CPUPPCState *env, target_ulong value,
     excp = 0;
     value &= env->msr_mask;
 #if !defined(CONFIG_USER_ONLY)
-    if (!alter_hv) {
-        /* mtmsr cannot alter the hypervisor state */
+    /* Neither mtmsr nor guest state can alter HV */
+    if (!alter_hv || !(env->msr & MSR_HVB)) {
         value &= ~MSR_HVB;
         value |= env->msr & MSR_HVB;
     }