scripts/gdb: add lx_current support for riscv

csr_sscratch CSR holds current task_struct address when hart is in user
space.  Trap handler on entry spills csr_sscratch into "tp" (x2) register
and zeroes out csr_sscratch CSR.  Trap handler on exit reloads "tp" with
expected user mode value and place current task_struct address again in
csr_sscratch CSR.

This patch assumes "tp" is pointing to task_struct. If value in
csr_sscratch is numerically greater than "tp" then it assumes csr_sscratch
is correct address of current task_struct. This logic holds when
   - hart is in user space, "tp" will be less than csr_sscratch.
   - hart is in kernel space but not in trap handler, "tp" will be more
     than csr_sscratch (csr_sscratch being equal to 0).
   - hart is executing trap handler
       - "tp" is still pointing to user mode but csr_sscratch contains
          ptr to task_struct. Thus numerically higher.
       - "tp" is  pointing to task_struct but csr_sscratch now contains
          either 0 or numerically smaller value (transiently holds
          user mode tp)

Link: https://lkml.kernel.org/r/20231026233837.612405-1-debug@rivosinc.com
Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Tested-by: Hsieh-Tseng Shen <woodrow.shen@sifive.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Glenn Washburn <development@efficientek.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Jeff Xie <xiehuan09@gmail.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Palmer Dabbelt <palmer@rivosinc.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: deepak0414

scripts/gdb/linux/cpus.py

@@ -179,6 +179,21 @@ def get_current_task(cpu):
         else:
             raise gdb.GdbError("Sorry, obtaining the current task is not allowed "
                                "while running in userspace(EL0)")
+    elif utils.is_target_arch("riscv"):
+        current_tp = gdb.parse_and_eval("$tp")
+        scratch_reg = gdb.parse_and_eval("$sscratch")
+
+        # by default tp points to current task
+        current_task = current_tp.cast(task_ptr_type)
+
+        # scratch register is set 0 in trap handler after entering kernel.
+        # When hart is in user mode, scratch register is pointing to task_struct.
+        # and tp is used by user mode. So when scratch register holds larger value
+        # (negative address as ulong is larger value) than tp, then use scratch register.
+        if (scratch_reg.cast(utils.get_ulong_type()) > current_tp.cast(utils.get_ulong_type())):
+            current_task = scratch_reg.cast(task_ptr_type)
+
+        return current_task.dereference()
     else:
         raise gdb.GdbError("Sorry, obtaining the current task is not yet "
                            "supported with this arch")