target/riscv: Expose "priv" register for GDB for reads

This patch enables a debugger to read the current privilege level via a virtual
"priv" register. When compiled with CONFIG_USER_ONLY the register is still
visible but always reports the value zero.

Signed-off-by: Jonathan Behrens <jonathan@fintelia.io>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

configure

@@ -7526,13 +7526,13 @@ case "$target_name" in
     TARGET_BASE_ARCH=riscv
     TARGET_ABI_DIR=riscv
     mttcg=yes
-    gdb_xml_files="riscv-32bit-cpu.xml riscv-32bit-fpu.xml riscv-32bit-csr.xml"
+    gdb_xml_files="riscv-32bit-cpu.xml riscv-32bit-fpu.xml riscv-32bit-csr.xml riscv-32bit-virtual.xml"
   ;;
   riscv64)
     TARGET_BASE_ARCH=riscv
     TARGET_ABI_DIR=riscv
     mttcg=yes
-    gdb_xml_files="riscv-64bit-cpu.xml riscv-64bit-fpu.xml riscv-64bit-csr.xml"
+    gdb_xml_files="riscv-64bit-cpu.xml riscv-64bit-fpu.xml riscv-64bit-csr.xml riscv-64bit-virtual.xml"
   ;;
   sh4|sh4eb)
     TARGET_ARCH=sh4

gdb-xml/riscv-32bit-virtual.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0"?>
+<!-- Copyright (C) 2018-2019 Free Software Foundation, Inc.
+
+     Copying and distribution of this file, with or without modification,
+     are permitted in any medium without royalty provided the copyright
+     notice and this notice are preserved.  -->
+
+<!DOCTYPE feature SYSTEM "gdb-target.dtd">
+<feature name="org.gnu.gdb.riscv.virtual">
+  <reg name="priv" bitsize="32"/>
+</feature>

gdb-xml/riscv-64bit-virtual.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0"?>
+<!-- Copyright (C) 2018-2019 Free Software Foundation, Inc.
+
+     Copying and distribution of this file, with or without modification,
+     are permitted in any medium without royalty provided the copyright
+     notice and this notice are preserved.  -->
+
+<!DOCTYPE feature SYSTEM "gdb-target.dtd">
+<feature name="org.gnu.gdb.riscv.virtual">
+  <reg name="priv" bitsize="64"/>
+</feature>

target/riscv/gdbstub.c

@@ -373,6 +373,23 @@ static int riscv_gdb_set_csr(CPURISCVState *env, uint8_t *mem_buf, int n)
     return 0;
 }
 
+static int riscv_gdb_get_virtual(CPURISCVState *cs, uint8_t *mem_buf, int n)
+{
+    if (n == 0) {
+#ifdef CONFIG_USER_ONLY
+        return gdb_get_regl(mem_buf, 0);
+#else
+        return gdb_get_regl(mem_buf, cs->priv);
+#endif
+    }
+    return 0;
+}
+
+static int riscv_gdb_set_virtual(CPURISCVState *cs, uint8_t *mem_buf, int n)
+{
+    return 0;
+}
+
 void riscv_cpu_register_gdb_regs_for_features(CPUState *cs)
 {
     RISCVCPU *cpu = RISCV_CPU(cs);
@@ -385,6 +402,9 @@ void riscv_cpu_register_gdb_regs_for_features(CPUState *cs)
 
     gdb_register_coprocessor(cs, riscv_gdb_get_csr, riscv_gdb_set_csr,
                              240, "riscv-32bit-csr.xml", 0);
+
+    gdb_register_coprocessor(cs, riscv_gdb_get_virtual, riscv_gdb_set_virtual,
+                             1, "riscv-32bit-virtual.xml", 0);
 #elif defined(TARGET_RISCV64)
     if (env->misa & RVF) {
         gdb_register_coprocessor(cs, riscv_gdb_get_fpu, riscv_gdb_set_fpu,
@@ -393,5 +413,8 @@ void riscv_cpu_register_gdb_regs_for_features(CPUState *cs)
 
     gdb_register_coprocessor(cs, riscv_gdb_get_csr, riscv_gdb_set_csr,
                              240, "riscv-64bit-csr.xml", 0);
+
+    gdb_register_coprocessor(cs, riscv_gdb_get_virtual, riscv_gdb_set_virtual,
+                             1, "riscv-64bit-virtual.xml", 0);
 #endif
 }