syscall_emul: add retry flag to SyscallReturn

This hook allows blocking emulated system calls to indicate
that they would block, but return control to the simulator
so that the simulation does not hang.  The actual retry
functionality requires additional support, to be provided
in a future changeset.

src/sim/syscall_emul.cc

@@ -71,10 +71,15 @@ SyscallDesc::doSyscall(int callnum, LiveProcess *process, ThreadContext *tc)
 
     SyscallReturn retval = (*funcPtr)(this, callnum, process, tc);
 
-    DPRINTFR(SyscallVerbose, "%d: %s: syscall %s returns %d\n",
-             curTick(), tc->getCpuPtr()->name(), name, retval.encodedValue());
+    if (retval.needsRetry()) {
+        DPRINTFS(SyscallVerbose, tc->getCpuPtr(), "syscall %s needs retry\n",
+                 name);
+    } else {
+        DPRINTFS(SyscallVerbose, tc->getCpuPtr(), "syscall %s returns %d\n",
+                 name, retval.encodedValue());
+    }
 
-    if (!(flags & SyscallDesc::SuppressReturnValue))
+    if (!(flags & SyscallDesc::SuppressReturnValue) && !retval.needsRetry())
         process->setSyscallReturn(tc, retval);
 }
 

src/sim/syscallreturn.hh

@@ -64,9 +64,17 @@ class SyscallReturn
     /// value is expected, e.g., as the return value from a system
     /// call emulation function ('return 0;' or 'return -EFAULT;').
     SyscallReturn(int64_t v)
-        : value(v)
+        : value(v), retryFlag(false)
     {}
 
+    /// Pseudo-constructor to create an instance with the retry flag set.
+    static SyscallReturn retry()
+    {
+        SyscallReturn s(0);
+        s.retryFlag = true;
+        return s;
+    }
+
     ~SyscallReturn() {}
 
     /// Was the system call successful?
@@ -75,6 +83,9 @@ class SyscallReturn
         return (value >= 0 || value <= -4096);
     }
 
+    /// Does the syscall need to be retried?
+    bool needsRetry() const { return retryFlag; }
+
     /// The return value
     int64_t returnValue() const
     {
@@ -98,6 +109,8 @@ class SyscallReturn
   private:
 
     int64_t value;
+
+    bool retryFlag;
 };
 
 #endif