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

1 files changed, 18 insertions, 4 deletions

diff --git a/target/arm/gdbstub.c b/target/arm/gdbstub.c
index 063551d..ecfa88f 100644
--- a/target/arm/gdbstub.c
+++ b/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.  */