base: Dump page table over RSP

Add a non-standard extension to the RSP protocol: the "." command
requests a dump of the simulated page table.
The dump consists of concatenated records, one record per page table
entry.  Each record contains the entry's "virtual" value written as
hex, followed by a colon (:), followed by the entry's "physical" value
written as hex, followed by a semicolon (;).

At the time of writing, one practical use of this feature (in
combination with the "shared_backstore" parameter) is extremely fast
Miranda-Ingalls simulation of JIT compilers.

Change-Id: I333ed11d4ce671251d0b93cddae3bbcea44ea4ca
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/47719
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/remote_gdb.cc

@@ -153,6 +153,7 @@
 #include "mem/port.hh"
 #include "mem/port_proxy.hh"
 #include "sim/full_system.hh"
+#include "sim/process.hh"
 #include "sim/system.hh"
 
 namespace gem5
@@ -872,6 +873,8 @@ std::map<char, BaseRemoteGDB::GdbCommand> BaseRemoteGDB::commandMap = {
     { 'z', { "KGDB_CLR_HW_BKPT", &BaseRemoteGDB::cmdClrHwBkpt } },
     // insert breakpoint or watchpoint
     { 'Z', { "KGDB_SET_HW_BKPT", &BaseRemoteGDB::cmdSetHwBkpt } },
+    // non-standard RSP extension: dump page table
+    { '.', { "GET_PAGE_TABLE", &BaseRemoteGDB::cmdDumpPageTable } },
 };
 
 bool
@@ -1218,6 +1221,13 @@ BaseRemoteGDB::encodeXferResponse(const std::string &unencoded,
     encodeBinaryData(unencoded.substr(offset, unencoded_length), encoded);
 }
 
+bool
+BaseRemoteGDB::cmdDumpPageTable(GdbCommand::Context &ctx)
+{
+    send(tc->getProcessPtr()->pTable->externalize().c_str());
+    return true;
+}
+
 bool
 BaseRemoteGDB::cmdAsyncStep(GdbCommand::Context &ctx)
 {

src/base/remote_gdb.hh

@@ -336,6 +336,7 @@ class BaseRemoteGDB
     bool cmdAsyncStep(GdbCommand::Context &ctx);
     bool cmdClrHwBkpt(GdbCommand::Context &ctx);
     bool cmdSetHwBkpt(GdbCommand::Context &ctx);
+    bool cmdDumpPageTable(GdbCommand::Context &ctx);
 
     struct QuerySetCommand
     {

src/mem/page_table.cc

@@ -206,4 +206,14 @@ EmulationPageTable::unserialize(CheckpointIn &cp)
     }
 }
 
+const std::string
+EmulationPageTable::externalize() const
+{
+    std::stringstream ss;
+    for (PTable::const_iterator it=pTable.begin(); it != pTable.end(); ++it) {
+        ss << std::hex << it->first << ":" << it->second.paddr << ";";
+    }
+    return ss.str();
+}
+
 } // namespace gem5

src/mem/page_table.hh

@@ -162,6 +162,12 @@ class EmulationPageTable : public Serializable
      */
     Fault translate(const RequestPtr &req);
 
+    /**
+     * Dump all items in the pTable, to a concatenation of strings of the form
+     *    Addr:Entry;
+     */
+    const std::string externalize() const;
+
     void getMappings(std::vector<std::pair<Addr, Addr>> *addr_mappings);
 
     void serialize(CheckpointOut &cp) const override;