syscall emulation: clean up & comment SyscallReturn

2 files changed, 56 insertions, 23 deletions

diff --git a/src/sim/syscall_emul.cc b/src/sim/syscall_emul.cc
index 935193e7f..61ba32955 100644
--- a/src/sim/syscall_emul.cc
+++ b/src/sim/syscall_emul.cc
@@ -69,7 +69,7 @@ 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.value());
+             curTick(), tc->getCpuPtr()->name(), name, retval.encodedValue());
 
     if (!(flags & SyscallDesc::SuppressReturnValue))
         process->setSyscallReturn(tc, retval);
@@ -366,7 +366,7 @@ readlinkFunc(SyscallDesc *desc, int num, LiveProcess *p, ThreadContext *tc,
     string path;
 
     if (!tc->getMemProxy().tryReadString(path, p->getSyscallArg(tc, index)))
-        return (TheISA::IntReg)-EFAULT;
+        return -EFAULT;
 
     // Adjust path for current working directory
     path = p->fullPath(path);
@@ -390,7 +390,7 @@ unlinkFunc(SyscallDesc *desc, int num, LiveProcess *p, ThreadContext *tc)
 
     int index = 0;
     if (!tc->getMemProxy().tryReadString(path, p->getSyscallArg(tc, index)))
-        return (TheISA::IntReg)-EFAULT;
+        return -EFAULT;
 
     // Adjust path for current working directory
     path = p->fullPath(path);
@@ -407,7 +407,7 @@ mkdirFunc(SyscallDesc *desc, int num, LiveProcess *p, ThreadContext *tc)
 
     int index = 0;
     if (!tc->getMemProxy().tryReadString(path, p->getSyscallArg(tc, index)))
-        return (TheISA::IntReg)-EFAULT;
+        return -EFAULT;
 
     // Adjust path for current working directory
     path = p->fullPath(path);
@@ -864,7 +864,7 @@ accessFunc(SyscallDesc *desc, int callnum, LiveProcess *p, ThreadContext *tc,
 {
     string path;
     if (!tc->getMemProxy().tryReadString(path, p->getSyscallArg(tc, index)))
-        return (TheISA::IntReg)-EFAULT;
+        return -EFAULT;
 
     // Adjust path for current working directory
     path = p->fullPath(path);
diff --git a/src/sim/syscallreturn.hh b/src/sim/syscallreturn.hh
index 385ff55db..547d76610 100644
--- a/src/sim/syscallreturn.hh
+++ b/src/sim/syscallreturn.hh
@@ -33,38 +33,71 @@
 
 #include "base/types.hh"
 
+/**
+ * This class represents the return value from an emulated system call,
+ * including any errno setting.
+ *
+ * On some platforms, the return value and errno are encoded in a
+ * single signed integer.  A value less than zero but greater than
+ * -4096 indicates an error, and the value is the negation of the
+ * errno value.  Otherwise, the call was successful and the integer is
+ * the return value.  (Large negative numbers are considered
+ * successful to allow syscalls to return pointers to high memory,
+ * e.g., stack addresses.)  See, for example, Appendix A of the AMD64
+ * ABI spec at http://www.x86-64.org/documentation/abi.pdf.
+ *
+ * Other platforms use a more complex interface, returning a value and
+ * an error code in separate registers.
+ *
+ * This class is designed to support both types of interfaces.
+ */
 class SyscallReturn
 {
   public:
-    template <class T>
-    SyscallReturn(T v, bool s)
+
+    /// For simplicity, allow the object to be initialized with a
+    /// single signed integer using the same positive=success,
+    /// negative=-errno convention described above.
+    ///
+    /// Typically this constructor is used as a default type
+    /// conversion, so a bare integer is used where a 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)
+    {}
+
+    ~SyscallReturn() {}
+
+    /// Was the system call successful?
+    bool successful() const
     {
-        retval = (uint64_t)v;
-        success = s;
+        return (value >= 0 || value <= -4096);
     }
 
-    template <class T>
-    SyscallReturn(T v)
+    /// The return value
+    int64_t returnValue() const
     {
-        success = (v >= 0);
-        retval = (uint64_t)v;
+        assert(successful());
+        return value;
     }
 
-    ~SyscallReturn() {}
+    /// The errno value
+    int errnoValue() const
+    {
+        assert(!successful());
+        return -value;
+    }
 
-    SyscallReturn& operator=(const SyscallReturn& s)
+    /// The encoded value (as described above)
+    int64_t encodedValue() const
     {
-        retval = s.retval;
-        success = s.success;
-        return *this;
+        return value;
     }
 
-    bool successful() { return success; }
-    uint64_t value() { return retval; }
+  private:
 
-    private:
-    uint64_t retval;
-    bool success;
+    int64_t value;
 };
 
 #endif