linux-user: Support for restarting system calls for tilegx targets

Update the tilegx main loop and sigreturn code:
 * on TARGET_ERESTARTSYS, wind guest PC backwards to repeat syscall insn
 * return -TARGET_QEMU_ESIGRETURN from sigreturn rather than current R_RE
 * handle TARGET_QEMU_ESIGRETURN in the main loop as the indication
   that the main loop should not touch any guest CPU state

Note that this fixes a bug where a sigreturn which happened to have
an errno value in TILEGX_R_RE would incorrectly cause TILEGX_R_ERR
to get set.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/main.c

@@ -3706,15 +3706,20 @@ void cpu_loop(CPUTLGState *env)
         cpu_exec_end(cs);
         switch (trapnr) {
         case TILEGX_EXCP_SYSCALL:
-            env->regs[TILEGX_R_RE] = do_syscall(env, env->regs[TILEGX_R_NR],
-                                                env->regs[0], env->regs[1],
-                                                env->regs[2], env->regs[3],
-                                                env->regs[4], env->regs[5],
-                                                env->regs[6], env->regs[7]);
-            env->regs[TILEGX_R_ERR] = TILEGX_IS_ERRNO(env->regs[TILEGX_R_RE])
-                                                      ? - env->regs[TILEGX_R_RE]
-                                                      : 0;
+        {
+            abi_ulong ret = do_syscall(env, env->regs[TILEGX_R_NR],
+                                       env->regs[0], env->regs[1],
+                                       env->regs[2], env->regs[3],
+                                       env->regs[4], env->regs[5],
+                                       env->regs[6], env->regs[7]);
+            if (ret == -TARGET_ERESTARTSYS) {
+                env->pc -= 8;
+            } else if (ret != -TARGET_QEMU_ESIGRETURN) {
+                env->regs[TILEGX_R_RE] = ret;
+                env->regs[TILEGX_R_ERR] = TILEGX_IS_ERRNO(ret) ? -ret : 0;
+            }
             break;
+        }
         case TILEGX_EXCP_OPCODE_EXCH:
             do_exch(env, true, false);
             break;

linux-user/signal.c

@@ -5709,7 +5709,7 @@ long do_rt_sigreturn(CPUTLGState *env)
     }
 
     unlock_user_struct(frame, frame_addr, 0);
-    return env->regs[TILEGX_R_RE];
+    return -TARGET_QEMU_ESIGRETURN;
 
 
  badframe:

linux-user/tilegx/target_signal.h

@@ -25,4 +25,5 @@ static inline abi_ulong get_sp_from_cpustate(CPUTLGState *state)
     return state->regs[TILEGX_R_SP];
 }
 
+
 #endif /* TARGET_SIGNAL_H */