target/arm: Use correct GDB XML for M-profile cores

GDB's remote protocol requires M-profile cores to use the feature
name 'org.gnu.gdb.arm.m-profile' instead of the 'org.gnu.gdb.arm.core'
feature used for A- and R-profile cores. We weren't doing this, which
meant GDB treated our M-profile cores like A-profile ones. This mostly
doesn't matter, but for instance means that it doesn't correctly
handle backtraces where an M-profile exception frame is involved.

Ship a copy of GDB's arm-m-profile.xml and use it on the M-profile
cores.  The integer registers have the same offsets as the
arm-core.xml, but register 25 is the M-profile XPSR rather than the
A-profile CPSR, so we need to update arm_cpu_gdb_read_register() and
arm_cpu_gdb_write_register() to handle XSPR reads and writes.

Fixes: https://bugs.launchpad.net/qemu/+bug/1877136
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20200507134755.13997-1-peter.maydell@linaro.org

configure

@@ -7806,14 +7806,14 @@ case "$target_name" in
     TARGET_SYSTBL_ABI=common,oabi
     bflt="yes"
     mttcg="yes"
-    gdb_xml_files="arm-core.xml arm-vfp.xml arm-vfp3.xml arm-neon.xml"
+    gdb_xml_files="arm-core.xml arm-vfp.xml arm-vfp3.xml arm-neon.xml arm-m-profile.xml"
   ;;
   aarch64|aarch64_be)
     TARGET_ARCH=aarch64
     TARGET_BASE_ARCH=arm
     bflt="yes"
     mttcg="yes"
-    gdb_xml_files="aarch64-core.xml aarch64-fpu.xml arm-core.xml arm-vfp.xml arm-vfp3.xml arm-neon.xml"
+    gdb_xml_files="aarch64-core.xml aarch64-fpu.xml arm-core.xml arm-vfp.xml arm-vfp3.xml arm-neon.xml arm-m-profile.xml"
   ;;
   cris)
   ;;

gdb-xml/arm-m-profile.xml

@@ -0,0 +1,27 @@
+<?xml version="1.0"?>
+<!-- Copyright (C) 2010-2020 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.arm.m-profile">
+  <reg name="r0" bitsize="32"/>
+  <reg name="r1" bitsize="32"/>
+  <reg name="r2" bitsize="32"/>
+  <reg name="r3" bitsize="32"/>
+  <reg name="r4" bitsize="32"/>
+  <reg name="r5" bitsize="32"/>
+  <reg name="r6" bitsize="32"/>
+  <reg name="r7" bitsize="32"/>
+  <reg name="r8" bitsize="32"/>
+  <reg name="r9" bitsize="32"/>
+  <reg name="r10" bitsize="32"/>
+  <reg name="r11" bitsize="32"/>
+  <reg name="r12" bitsize="32"/>
+  <reg name="sp" bitsize="32" type="data_ptr"/>
+  <reg name="lr" bitsize="32"/>
+  <reg name="pc" bitsize="32" type="code_ptr"/>
+  <reg name="xpsr" bitsize="32" regnum="25"/>
+</feature>

target/arm/cpu_tcg.c

@@ -605,6 +605,7 @@ static void arm_v7m_class_init(ObjectClass *oc, void *data)
 #endif
 
     cc->cpu_exec_interrupt = arm_v7m_cpu_exec_interrupt;
+    cc->gdb_core_xml_file = "arm-m-profile.xml";
 }
 
 static const ARMCPUInfo arm_tcg_cpus[] = {

target/arm/gdbstub.c

@@ -57,8 +57,12 @@ int arm_cpu_gdb_read_register(CPUState *cs, GByteArray *mem_buf, int n)
         }
         return gdb_get_reg32(mem_buf, 0);
     case 25:
-        /* CPSR */
-        return gdb_get_reg32(mem_buf, cpsr_read(env));
+        /* CPSR, or XPSR for M-profile */
+        if (arm_feature(env, ARM_FEATURE_M)) {
+            return gdb_get_reg32(mem_buf, xpsr_read(env));
+        } else {
+            return gdb_get_reg32(mem_buf, cpsr_read(env));
+        }
     }
     /* Unknown register.  */
     return 0;
@@ -98,8 +102,18 @@ int arm_cpu_gdb_write_register(CPUState *cs, uint8_t *mem_buf, int n)
         }
         return 4;
     case 25:
-        /* CPSR */
-        cpsr_write(env, tmp, 0xffffffff, CPSRWriteByGDBStub);
+        /* CPSR, or XPSR for M-profile */
+        if (arm_feature(env, ARM_FEATURE_M)) {
+            /*
+             * Don't allow writing to XPSR.Exception as it can cause
+             * a transition into or out of handler mode (it's not
+             * writeable via the MSR insn so this is a reasonable
+             * restriction). Other fields are safe to update.
+             */
+            xpsr_write(env, tmp, ~XPSR_EXCP);
+        } else {
+            cpsr_write(env, tmp, 0xffffffff, CPSRWriteByGDBStub);
+        }
         return 4;
     }
     /* Unknown register.  */