Re: [PATCH 2/4] Fall back to a default value of 0 for the MISA register.

[John Baldwin <jhb@FreeBSD.org>]

On 9/19/18 5:40 PM, John Baldwin wrote:
> Whether or not 'C' is present is a bit more subtle as it isn't something
> you could infer just from a register being present or not.  On the other
> hand, for breakpoints, I wonder if a more straightforward approach would
> be to examine the opcode of the existing instruction and use that to
> decide which breakpoint to use?  That is, read the first byte of the
> existing instruction and if the low 2 bits indicate a 16-bit instruction
> use C.BREAK and otherwise use BREAK?

This seems to work for me in my testing (I used 'stepi' to step across a
set of instructions with a mix of regular and 'C' instructions and it
worked correctly).  You can use 'set debug riscv breakpoints 1' to see what
it decides each time it sets a breakpoint.  I haven't yet tested if this
means I can remove the MISA patch, but I suspect it does as the only
place that needs MISA after this is the riscv_isa_flen() function.

Author: John Baldwin <jhb@FreeBSD.org>
Date:   Thu Sep 20 10:12:22 2018 -0700

    Use the existing instruction to determine the RISC-V breakpoint kind.
    
    RISC-V supports instructions of varying lengths.  Standard existing
    instructions in the base ISA are 4 bytes in length, but the 'C'
    extension adds support for compressed, 2 byte instructions.  RISC-V
    supports two different breakpoint instructions: EBREAK is a 4 byte
    instruction in the base ISA, and C.EBREAK is a 2 byte instruction only
    available on processors implementing the 'C' extension.  Using EBREAK
    to set breakpoints on compressed instructions causes problems as the
    second half of EBREAK will overwrite the first 2 bytes of the
    following instruction breaking other threads in the process if their
    PC is the following instruction.  Thus, breakpoints on compressed
    instructions need to use C.EBREAK instead of EBREAK.
    
    Previously, the riscv architecture checked the MISA register to
    determine if the 'C' extension was available.  If so, it used C.EBREAK
    for all breakpoints.  However, the MISA register is not necessarily
    available to supervisor mode operating systems.  While native targets
    could provide a fake MISA register value, this patch instead examines
    the existing instruction at a breakpoint target to determine which
    breakpoint instruction to use.  If the existing instruction is a
    compressed instruction, C.EBREAK is used, otherwise EBREAK is used.
    
    gdb/ChangeLog:
    
            * riscv-tdep.c (show_use_compressed_breakpoints): Remove
            'additional_info' and related logic.
            (riscv_debug_breakpoints): New variable.
            (riscv_breakpoint_kind_from_pc): Use the length of the existing
            instruction to determine the breakpoint kind.
            (_initialize_riscv_tdep): Add 'set/show debug riscv breakpoints'
            flag.  Update description of 'set/show riscv
            use-compressed-breakpoints' flag.

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 75032b7a4d..df4561d319 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,14 @@
+2018-09-20  John Baldwin  <jhb@FreeBSD.org>
+
+	* riscv-tdep.c (show_use_compressed_breakpoints): Remove
+	'additional_info' and related logic.
+	(riscv_debug_breakpoints): New variable.
+	(riscv_breakpoint_kind_from_pc): Use the length of the existing
+	instruction to determine the breakpoint kind.
+	(_initialize_riscv_tdep): Add 'set/show debug riscv breakpoints'
+	flag.  Update description of 'set/show riscv
+	use-compressed-breakpoints' flag.
+
 2018-09-19  John Baldwin  <jhb@FreeBSD.org>
 
 	* Makefile.in (ALLDEPFILES): Add riscv-fbsd-nat.c.
diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
index 4b385ed5da..cd8dbaf9f6 100644
--- a/gdb/riscv-tdep.c
+++ b/gdb/riscv-tdep.c
@@ -210,20 +210,9 @@ show_use_compressed_breakpoints (struct ui_file *file, int from_tty,
 				 struct cmd_list_element *c,
 				 const char *value)
 {
-  const char *additional_info;
-  struct gdbarch *gdbarch = target_gdbarch ();
-
-  if (use_compressed_breakpoints == AUTO_BOOLEAN_AUTO)
-    if (riscv_has_feature (gdbarch, 'C'))
-      additional_info = _(" (currently on)");
-    else
-      additional_info = _(" (currently off)");
-  else
-    additional_info = "";
-
   fprintf_filtered (file,
 		    _("Debugger's use of compressed breakpoints is set "
-		      "to %s%s.\n"), value, additional_info);
+		      "to %s.\n"), value);
 }
 
 /* The set and show lists for 'set riscv' and 'show riscv' prefixes.  */
@@ -284,6 +273,11 @@ show_riscv_debug_variable (struct ui_file *file, int from_tty,
 		    c->name, value);
 }
 
+/* When this is set to non-zero debugging information about breakpoint
+   kinds will be printed.  */
+
+static unsigned int riscv_debug_breakpoints = 0;
+
 /* When this is set to non-zero debugging information about inferior calls
    will be printed.  */
 
@@ -426,7 +420,22 @@ riscv_breakpoint_kind_from_pc (struct gdbarch *gdbarch, CORE_ADDR *pcptr)
 {
   if (use_compressed_breakpoints == AUTO_BOOLEAN_AUTO)
     {
-      if (riscv_has_feature (gdbarch, 'C'))
+      enum bfd_endian byte_order = gdbarch_byte_order_for_code (gdbarch);
+      gdb_byte buf[1];
+      int status;
+
+      /* Read the opcode byte to determine the instruction length.  */
+      status = target_read_memory (*pcptr, buf, 1);
+      if (status)
+	memory_error (TARGET_XFER_E_IO, *pcptr);
+
+      if (riscv_debug_breakpoints)
+	fprintf_unfiltered
+	  (gdb_stdlog,
+	   "Using %s for breakpoint at %s (instruction length %d)\n",
+	   riscv_insn_length (buf[0]) == 2 ? "C.EBREAK" : "EBREAK",
+	   paddress (gdbarch, *pcptr), riscv_insn_length (buf[0]));
+      if (riscv_insn_length (buf[0]) == 2)
 	return 2;
       else
 	return 4;
@@ -2945,6 +2954,16 @@ _initialize_riscv_tdep (void)
 		  &showdebugriscvcmdlist, "show debug riscv ", 0,
 		  &showdebuglist);
 
+  add_setshow_zuinteger_cmd ("breakpoints", class_maintenance,
+			     &riscv_debug_breakpoints,  _("\
+Set riscv breakpoint debugging."), _("\
+Show riscv breakpoint debugging."), _("\
+When non-zero, print debugging information for the riscv specific parts\n\
+of the breakpoint mechanism."),
+			     NULL,
+			     show_riscv_debug_variable,
+			     &setdebugriscvcmdlist, &showdebugriscvcmdlist);
+
   add_setshow_zuinteger_cmd ("infcall", class_maintenance,
 			     &riscv_debug_infcall,  _("\
 Set riscv inferior call debugging."), _("\
@@ -2982,9 +3001,9 @@ of the stack unwinding mechanism."),
 Set debugger's use of compressed breakpoints."), _("	\
 Show debugger's use of compressed breakpoints."), _("\
 Debugging compressed code requires compressed breakpoints to be used. If\n \
-left to 'auto' then gdb will use them if $misa indicates the C extension\n \
-is supported. If that doesn't give the correct behavior, then this option\n\
-can be used."),
+left to 'auto' then gdb will use them if the existing instruction is a\n \
+compressed instruction. If that doesn't give the correct behavior, then\n \
+this option can be used."),
 				NULL,
 				show_use_compressed_breakpoints,
 				&setriscvcmdlist,

-- 
John Baldwin