target-arm: allow modifying vfp fpexc en bit only

All other bits except for the EN in the VFP FPEXC register are defined
as subarchitecture specific and real functionality for any of the
other bits has not been implemented in QEMU. However, current code
allows modifying all bits in the VFP FPEXC register leading to
problems when guest code is writing 1's to the subarchitecture
specific bits and checking whether the bits stay up to verify the
existence of functionality which in fact does not exist in QEMU.
This patch has been revised to include the same behavior change in
the gdb register write function.
Signed-off-by: NJuha Riihimäki <juha.riihimaki@nokia.com>
Acked-by: NLaurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

target-arm/helper.c

@@ -234,7 +234,7 @@ static int vfp_gdb_set_reg(CPUState *env, uint8_t *buf, int reg)
     switch (reg - nregs) {
     case 0: env->vfp.xregs[ARM_VFP_FPSID] = ldl_p(buf); return 4;
     case 1: env->vfp.xregs[ARM_VFP_FPSCR] = ldl_p(buf); return 4;
-    case 2: env->vfp.xregs[ARM_VFP_FPEXC] = ldl_p(buf); return 4;
+    case 2: env->vfp.xregs[ARM_VFP_FPEXC] = ldl_p(buf) & (1 << 30); return 4;
     }
     return 0;
 }

target-arm/translate.c

@@ -2788,6 +2788,9 @@ static int disas_vfp_insn(CPUState * env, DisasContext *s, uint32_t insn)
                         case ARM_VFP_FPEXC:
                             if (IS_USER(s))
                                 return 1;
+                            /* TODO: VFP subarchitecture support.
+                             * For now, keep the EN bit only */
+                            tcg_gen_andi_i32(tmp, tmp, 1 << 30);
                             store_cpu_field(tmp, vfp.xregs[rn]);
                             gen_lookup_tb(s);
                             break;