[PATCH v2] gdbserver: select a thread, if necessary, to access memory (was: [PATCH] gdbserver: try selecting a thread first to access memory)

[Tankut Baris Aktemur <tankut.baris.aktemur@intel.com>]

Since commit

  commit 7f8acedeebe295fc8cc1d11ed971cbfc1942618c
  Author: Pedro Alves <pedro@palves.net>
  Date:   Tue Apr 5 13:57:11 2022 +0100

    gdbserver: track current process as well as current thread

gdbserver switches to a process, rather than a thread, before
processing a memory access request coming from the GDB side.  This
switch sets current_thread to null.  Some memory accesses on certain
targets, however, may require having a thread context.  Therefore,
switch to the selected thread, if the target would require a thread
context.
---
 gdbserver/server.cc | 16 ++++++++++++++--
 gdbserver/target.cc |  6 ++++++
 gdbserver/target.h  |  7 +++++++
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/gdbserver/server.cc b/gdbserver/server.cc
index c57270175b4..44642a64215 100644
--- a/gdbserver/server.cc
+++ b/gdbserver/server.cc
@@ -1072,7 +1072,13 @@ gdb_read_memory (CORE_ADDR memaddr, unsigned char *myaddr, int len)
       /* (assume no half-trace half-real blocks for now) */
     }
 
-  if (set_desired_process ())
+  /* Some memory accesses may require having a thread context.
+     Attempt switching to the selected thread, if necessary.
+     Otherwise use the process as the context.  */
+  bool need_thread = the_target->memaddr_is_thread_specific (memaddr);
+
+  if ((need_thread && set_desired_thread ())
+      || (!need_thread && set_desired_process ()))
     res = read_inferior_memory (memaddr, myaddr, len);
   else
     res = 1;
@@ -1093,7 +1099,13 @@ gdb_write_memory (CORE_ADDR memaddr, const unsigned char *myaddr, int len)
     {
       int ret;
 
-      if (set_desired_process ())
+      /* Some memory accesses may require having a thread context.
+	 Attempt switching to the selected thread, if necessary.
+	 Otherwise use the process as the context.  */
+      bool need_thread = the_target->memaddr_is_thread_specific (memaddr);
+
+      if ((need_thread && set_desired_thread ())
+	  || (!need_thread && set_desired_process ()))
 	ret = target_write_memory (memaddr, myaddr, len);
       else
 	ret = EIO;
diff --git a/gdbserver/target.cc b/gdbserver/target.cc
index f8e592d20c3..85efe6d88c6 100644
--- a/gdbserver/target.cc
+++ b/gdbserver/target.cc
@@ -832,3 +832,9 @@ process_stratum_target::get_ipa_tdesc_idx ()
 {
   return 0;
 }
+
+bool
+process_stratum_target::memaddr_is_thread_specific (CORE_ADDR memaddr)
+{
+  return false;
+}
diff --git a/gdbserver/target.h b/gdbserver/target.h
index d993e361b76..bde2fb2eb4c 100644
--- a/gdbserver/target.h
+++ b/gdbserver/target.h
@@ -508,6 +508,13 @@ class process_stratum_target
      Returns true if successful and false otherwise.  */
   virtual bool store_memtags (CORE_ADDR address, size_t len,
 			      const gdb::byte_vector &tags, int type);
+
+  /* Some memory addresses may require having a thread context.  E.g.:
+     an address whose upper bits encode a thread-specific address
+     space.  Let the target tell if MEMADDR is such an address, so
+     that the server can attempt switching the context before
+     accessing the memory.  */
+  virtual bool memaddr_is_thread_specific (CORE_ADDR memaddr);
 };
 
 extern process_stratum_target *the_target;
-- 
2.34.1

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