gdb/riscv: write CSRs into baremetal core dumps

Use the current target description to include CSRs into the RISC-V
baremetal core dumps.

Every CSR declared in the current target description will be included
in the core dump.

It will be critical for users that they have the same target
description in use when loading the core file as was in use when
writing the core file.  This should be fine if the user allows the
target description to be written into the core file.

In more detail, this commit adds a NT_RISCV_CSR note type.  The
contents of this section is a series of either 4-byte (on RV32
targets), or 8-byte (on RV64 targets) values.  Every CSR that is
mentioned in the current target description is written out in the
order the registers appear in the target description.  As a
consequence it is critical that the exact same target description,
including the same register order, is in use when the CSRs are loaded
from the core file.

gdb/ChangeLog:

	* riscv-none-tdep.c: Add 'user-regs.h' and 'target-description.h'
	includes.
	(riscv_csrset): New static global.
	(riscv_update_csrmap): New function.
	(riscv_iterate_over_regset_sections): Process CSRs.

gdb/ChangeLog

@@ -1,3 +1,12 @@
+2021-03-05  Craig Blackmore  <craig.blackmore@embecosm.com>
+	    Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* riscv-none-tdep.c: Add 'user-regs.h' and 'target-description.h'
+	includes.
+	(riscv_csrset): New static global.
+	(riscv_update_csrmap): New function.
+	(riscv_iterate_over_regset_sections): Process CSRs.
+
 2021-03-05  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* riscv-tdep.c (riscv_feature_name_csr): Define.

gdb/riscv-none-tdep.c

@@ -23,6 +23,8 @@
 #include "riscv-tdep.h"
 #include "elf-bfd.h"
 #include "regset.h"
+#include "user-regs.h"
+#include "target-descriptions.h"
 
 #ifdef HAVE_ELF
 #include "elf-none-tdep.h"
@@ -65,6 +67,42 @@ static const struct regset riscv_fregset =
   riscv_fregmap, riscv_supply_regset, regcache_collect_regset
 };
 
+/* Define the CSR regset, this is not constant as the regmap field is
+   updated dynamically based on the current target description.  */
+
+static struct regset riscv_csrset =
+{
+  nullptr, regcache_supply_regset, regcache_collect_regset
+};
+
+/* Update the regmap field of RISCV_CSRSET based on the CSRs available in
+   the current target description.  */
+
+static void
+riscv_update_csrmap (struct gdbarch *gdbarch,
+		     const struct tdesc_feature *feature_csr)
+{
+  int i = 0;
+
+  /* Release any previously defined map.  */
+  delete[] ((struct regcache_map_entry *) riscv_csrset.regmap);
+
+  /* Now create a register map for every csr found in the target
+     description.  */
+  struct regcache_map_entry *riscv_csrmap
+    = new struct regcache_map_entry[feature_csr->registers.size() + 1];
+  for (auto &csr : feature_csr->registers)
+    {
+      int regnum = user_reg_map_name_to_regnum (gdbarch, csr->name.c_str(),
+						csr->name.length());
+      riscv_csrmap[i++] = {1, regnum, 0};
+    }
+
+  /* Mark the end of the array.  */
+  riscv_csrmap[i] = {0};
+  riscv_csrset.regmap = riscv_csrmap;
+}
+
 /* Implement the "iterate_over_regset_sections" gdbarch method.  */
 
 static void
@@ -84,6 +122,28 @@ riscv_iterate_over_regset_sections (struct gdbarch *gdbarch,
 	    + register_size (gdbarch, RISCV_CSR_FCSR_REGNUM));
       cb (".reg2", sz, sz, &riscv_fregset, NULL, cb_data);
     }
+
+  /* Read or write the CSRs.  The set of CSRs is defined by the current
+     target description.  The user is responsible for ensuring that the
+     same target description is in use when reading the core file as was
+     in use when writing the core file.  */
+  const struct target_desc *tdesc = gdbarch_target_desc (gdbarch);
+
+  /* Do not dump/load any CSRs if there is no target description or the target
+     description does not contain any CSRs.  */
+  if (tdesc != nullptr)
+    {
+      const struct tdesc_feature *feature_csr
+        = tdesc_find_feature (tdesc, riscv_feature_name_csr);
+      if (feature_csr != nullptr && feature_csr->registers.size () > 0)
+	{
+	  riscv_update_csrmap (gdbarch, feature_csr);
+	  cb (".reg-riscv-csr",
+	      (feature_csr->registers.size() * riscv_isa_xlen (gdbarch)),
+	      (feature_csr->registers.size() * riscv_isa_xlen (gdbarch)),
+	      &riscv_csrset, NULL, cb_data);
+	}
+    }
 }
 
 /* Initialize RISC-V bare-metal ABI info.  */