sim: Trap into GDB instead of panicking on SEGV

When a segfault happens in the guest, report a SEGV trap to GDB (if
there is one attached) instead of bailing out immediately.

The obvious use-case for this, is the ability to debug guest crashes
in GDB in the standard manner.

The less-trivial use-case is for development of software in an
incomplete software stack (cf. Aarno-Engblom's "Virtual Platforms"
pp.105 et seq.)  One particular example is Ingalls-Miranda simulation of
JIT compilers, where the VM's address space may be split between the
simulated and the real machine: in this case, GDB traps facilitate the
transparent illusion of an unbroken address space.

Change-Id: I9072ed5f6474e05e9a99dc42ae5754be28121355
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/44685
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/null/remote_gdb.hh

@@ -47,6 +47,7 @@ class BaseRemoteGDB
 
     bool breakpoint() { return false; }
     void replaceThreadContext(ThreadContext *tc) {}
+    bool trap(int type) { return true; }
 
     virtual ~BaseRemoteGDB() {}
 };

src/sim/faults.cc

@@ -40,6 +40,8 @@
 
 #include "sim/faults.hh"
 
+#include <csignal>
+
 #include "arch/decoder.hh"
 #include "arch/locked_mem.hh"
 #include "base/logging.hh"
@@ -94,15 +96,16 @@ GenericPageTableFault::invoke(ThreadContext *tc, const StaticInstPtr &inst)
         Process *p = tc->getProcessPtr();
         handled = p->fixupFault(vaddr);
     }
-    panic_if(!handled, "Page table fault when accessing virtual address %#x",
-             vaddr);
-
+    panic_if(!handled &&
+                 !tc->getSystemPtr()->trapToGdb(SIGSEGV, tc->contextId()),
+             "Page table fault when accessing virtual address %#x\n", vaddr);
 }
 
 void
 GenericAlignmentFault::invoke(ThreadContext *tc, const StaticInstPtr &inst)
 {
-    panic("Alignment fault when accessing virtual address %#x\n", vaddr);
+    panic_if(!tc->getSystemPtr()->trapToGdb(SIGSEGV, tc->contextId()),
+             "Alignment fault when accessing virtual address %#x\n", vaddr);
 }
 
 void GenericHtmFailureFault::invoke(ThreadContext *tc,

src/sim/system.cc

@@ -495,6 +495,16 @@ System::workItemEnd(uint32_t tid, uint32_t workid)
     lastWorkItemStarted.erase(p);
 }
 
+bool
+System::trapToGdb(int signal, ContextID ctx_id) const
+{
+    auto *gdb = threads.thread(ctx_id).gdb;
+    if (!gdb)
+        return false;
+    gdb->trap(signal);
+    return true;
+}
+
 void
 System::printSystems()
 {

src/sim/system.hh

@@ -561,6 +561,9 @@ class System : public SimObject, public PCEventScope
 
     void workItemEnd(uint32_t tid, uint32_t workid);
 
+    /* Returns whether we successfully trapped into GDB. */
+    bool trapToGdb(int signal, ContextID ctx_id) const;
+
   protected:
     /**
      * Range for memory-mapped m5 pseudo ops. The range will be