RISC-V/Linux/native: Factor out target description determination

In preparation for RISC-V/Linux `gdbserver' support factor out parts of
native target description determination code that can be shared between
the programs.

	gdb/
	* nat/riscv-linux-tdesc.h: New file.
	* nat/riscv-linux-tdesc.c: New file, taking code from...
	* riscv-linux-nat.c (riscv_linux_nat_target::read_description):
	... here.
	* configure.nat <linux> <riscv*>: Add nat/riscv-linux-tdesc.o to
	NATDEPFILES.

gdb/ChangeLog

@@ -1,3 +1,12 @@
+2020-02-05  Maciej W. Rozycki  <macro@wdc.com>
+
+	* nat/riscv-linux-tdesc.h: New file.
+	* nat/riscv-linux-tdesc.c: New file, taking code from...
+	* riscv-linux-nat.c (riscv_linux_nat_target::read_description):
+	... here.
+	* configure.nat <linux> <riscv*>: Add nat/riscv-linux-tdesc.o to
+	NATDEPFILES.
+
 2020-02-04  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* remote-sim.c (sim_inferior_data::sim_inferior_data): Assert that

gdb/configure.nat

@@ -276,7 +276,8 @@ case ${gdb_host} in
 		;;
 	    riscv*)
 		# Host: RISC-V, running Linux
-		NATDEPFILES="${NATDEPFILES} riscv-linux-nat.o"
+		NATDEPFILES="${NATDEPFILES} riscv-linux-nat.o \
+		nat/riscv-linux-tdesc.o"
 		;;
 	    s390)
 		# Host: S390, running Linux

gdb/nat/riscv-linux-tdesc.c

@@ -0,0 +1,83 @@
+/* GNU/Linux/RISC-V native target description support for GDB.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "gdbsupport/common-defs.h"
+
+#include "gdb_proc_service.h"
+#include "arch/riscv.h"
+#include "elf/common.h"
+#include "nat/gdb_ptrace.h"
+#include "nat/riscv-linux-tdesc.h"
+
+#include <sys/uio.h>
+
+/* Work around glibc header breakage causing ELF_NFPREG not to be usable.  */
+#ifndef NFPREG
+# define NFPREG 33
+#endif
+
+/* Determine XLEN and FLEN and return a corresponding target description.  */
+
+const struct target_desc *
+riscv_linux_read_description (int tid)
+{
+  struct riscv_gdbarch_features features;
+  elf_fpregset_t regs;
+  int flen;
+
+  /* Figuring out xlen is easy.  */
+  features.xlen = sizeof (elf_greg_t);
+
+  /* Start with no f-registers.  */
+  features.flen = 0;
+
+  /* How much worth of f-registers can we fetch if any?  */
+  for (flen = sizeof (regs.__f.__f[0]); ; flen *= 2)
+    {
+      size_t regset_size;
+      struct iovec iov;
+
+      /* Regsets have a uniform slot size, so we count FSCR like
+	 an FP data register.  */
+      regset_size = ELF_NFPREG * flen;
+      if (regset_size > sizeof (regs))
+	break;
+
+      iov.iov_base = &regs;
+      iov.iov_len = regset_size;
+      if (ptrace (PTRACE_GETREGSET, tid, NT_FPREGSET,
+		  (PTRACE_TYPE_ARG3) &iov) == -1)
+	{
+	  switch (errno)
+	    {
+	    case EINVAL:
+	      continue;
+	    case EIO:
+	      break;
+	    default:
+	      perror_with_name (_("Couldn't get registers"));
+	      break;
+	    }
+	}
+      else
+	features.flen = flen;
+      break;
+    }
+
+  return riscv_create_target_description (features);
+}

gdb/nat/riscv-linux-tdesc.h

@@ -0,0 +1,27 @@
+/* GNU/Linux/RISC-V native target description support for GDB.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef NAT_RISCV_LINUX_TDESC_H
+#define NAT_RISCV_LINUX_TDESC_H
+
+struct target_desc;
+
+/* Return a target description for the LWP identified by TID.  */
+const struct target_desc *riscv_linux_read_description (int tid);
+
+#endif /* NAT_RISCV_LINUX_TDESC_H */

gdb/riscv-linux-nat.c

@@ -22,10 +22,11 @@
 #include "linux-nat.h"
 #include "riscv-tdep.h"
 #include "inferior.h"
-#include "target-descriptions.h"
 
 #include "elf/common.h"
 
+#include "nat/riscv-linux-tdesc.h"
+
 #include <sys/ptrace.h>
 
 /* Work around glibc header breakage causing ELF_NFPREG not to be usable.  */
@@ -200,53 +201,7 @@ fill_fpregset (const struct regcache *regcache, prfpregset_t *fpregs,
 const struct target_desc *
 riscv_linux_nat_target::read_description ()
 {
-  struct riscv_gdbarch_features features;
+  return riscv_linux_read_description (inferior_ptid.lwp ());
-  elf_fpregset_t regs;
-  int flen;
-  int tid;
-
-  /* Figuring out xlen is easy.  */
-  features.xlen = sizeof (elf_greg_t);
-
-  tid = inferior_ptid.lwp ();
-
-  /* Start with no f-registers.  */
-  features.flen = 0;
-
-  /* How much worth of f-registers can we fetch if any?  */
-  for (flen = sizeof (regs.__f.__f[0]); ; flen *= 2)
-    {
-      size_t regset_size;
-      struct iovec iov;
-
-      /* Regsets have a uniform slot size, so we count FSCR like
-	 an FP data register.  */
-      regset_size = ELF_NFPREG * flen;
-      if (regset_size > sizeof (regs))
-	break;
-
-      iov.iov_base = &regs;
-      iov.iov_len = regset_size;
-      if (ptrace (PTRACE_GETREGSET, tid, NT_FPREGSET,
-		  (PTRACE_TYPE_ARG3) &iov) == -1)
-	{
-	  switch (errno)
-	    {
-	    case EINVAL:
-	      continue;
-	    case EIO:
-	      break;
-	    default:
-	      perror_with_name (_("Couldn't get registers"));
-	      break;
-	    }
-	}
-      else
-	features.flen = flen;
-      break;
-    }
-
-  return riscv_create_target_description (features);
 }
 
 /* Fetch REGNUM (or all registers if REGNUM == -1) from the target