sim-se: (Re)add support for retrying system calls.

The previous incarnation of this support used faults to make the CPU
reexecute the system call instruction again and again to prevent
emulating/passing through blocking system calls from blocking gem5 as
a whole. That support was accidentally removed a while ago. This new
version suspends the thread context executing the system call, and
periodically wakes it up to retry using a periodically scheduled event.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-1123

Change-Id: I4fad23bd119021df23fe6082891fbcd0ee8fb839
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/54024
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/syscall_desc.cc

@@ -30,6 +30,7 @@
 #include "sim/syscall_desc.hh"
 
 #include "base/types.hh"
+#include "sim/eventq.hh"
 #include "sim/syscall_debug_macros.hh"
 
 namespace gem5
@@ -45,12 +46,58 @@ SyscallDesc::doSyscall(ThreadContext *tc)
     SyscallReturn retval = executor(this, tc);
 
     if (retval.needsRetry()) {
-        DPRINTF_SYSCALL(Base, "Needs retry.\n", name());
-    } else if (retval.suppressed()) {
+        // Suspend this ThreadContext while the syscall is pending.
+        tc->suspend();
+
+        DPRINTF_SYSCALL(Base, "%s needs retry.\n", name());
+        setupRetry(tc);
+        return;
+    }
+
+    handleReturn(tc, retval);
+}
+
+void
+SyscallDesc::retrySyscall(ThreadContext *tc)
+{
+    DPRINTF_SYSCALL(Base, "Retrying %s...\n", dumper(name(), tc));
+
+    SyscallReturn retval = executor(this, tc);
+
+    if (retval.needsRetry()) {
+        DPRINTF_SYSCALL(Base, "%s still needs retry.\n", name());
+        setupRetry(tc);
+        return;
+    }
+
+    // We're done retrying, so reactivate this ThreadContext.
+    tc->activate();
+
+    handleReturn(tc, retval);
+}
+
+void
+SyscallDesc::setupRetry(ThreadContext *tc)
+{
+    // Create an event which will retry the system call later.
+    auto retry = [this, tc]() { retrySyscall(tc); };
+    auto *event = new EventFunctionWrapper(retry, name(), true);
+
+    // Schedule it in about 100 CPU cycles. That will give other contexts
+    // a chance to execute a bit of code before trying again.
+    auto *cpu = tc->getCpuPtr();
+    curEventQueue()->schedule(event,
+            curTick() + cpu->cyclesToTicks(Cycles(100)));
+}
+
+void
+SyscallDesc::handleReturn(ThreadContext *tc, const SyscallReturn &ret)
+{
+    if (ret.suppressed()) {
         DPRINTF_SYSCALL(Base, "No return value.\n", name());
     } else {
-        returnInto(tc, retval);
-        DPRINTF_SYSCALL(Base, "Returned %d.\n", retval.encodedValue());
+        returnInto(tc, ret);
+        DPRINTF_SYSCALL(Base, "Returned %d.\n", ret.encodedValue());
     }
 }
 

src/sim/syscall_desc.hh

@@ -95,11 +95,16 @@ class SyscallDesc
         _name(name), _num(num), executor(exec), dumper(dump)
     {}
 
+    void retrySyscall(ThreadContext *tc);
+
   private:
     /** System call name (e.g., open, mmap, clone, socket, etc.) */
     std::string _name;
     int _num;
 
+    void setupRetry(ThreadContext *tc);
+    void handleReturn(ThreadContext *tc, const SyscallReturn &ret);
+
     /** Mechanism for ISAs to connect to the emul function definitions */
     Executor executor;
     Dumper dumper;