[Qemu-devel] [PATCH] gdbstub: allow byte swapping for reading/writing registers

[Thomas Falcon <tlfalcon@linux.vnet.ibm.com>]

This patch allows registers to be properly read from and written to
when using the gdbstub to debug a ppc guest running in little
endian mode.  It accomplishes this goal by byte swapping the values of
any registers only if the MSR:LE value is set and if the host machine
is big endian.

Signed-off-by: Thomas Falcon<tlfalcon@linux.vnet.ibm.com>
---
  target-ppc/gdbstub.c | 50 ++++++++++++++++++++++++++++++++------------------
  1 file changed, 32 insertions(+), 18 deletions(-)

diff --git a/target-ppc/gdbstub.c b/target-ppc/gdbstub.c
index 1c91090..eba501a 100644
--- a/target-ppc/gdbstub.c
+++ b/target-ppc/gdbstub.c
@@ -21,6 +21,19 @@
  #include "qemu-common.h"
  #include "exec/gdbstub.h"

+/* The following macros are used to ensure the correct
+ * transfer of registers between a little endian ppc target
+ * and a big endian host by checking the LE bit in the Machine State Register
+ */
+
+#define end_swap64(x) (msr_le && HOST_WORDS_BIGENDIAN) ? bswap64(x) : x
+#define end_swap32(x) (msr_le && HOST_WORDS_BIGENDIAN) ? bswap32(x) : x
+#if TARGET_LONG_BITS == 64
+#define end_swapl(x) end_swap64(x)
+#else
+#define end_swapl(x) end_swap32(x)
+#endif
+
  /* Old gdb always expects FP registers.  Newer (xml-aware) gdb only
   * expects whatever the target description contains.  Due to a
   * historical mishap the FP registers appear in between core integer
@@ -35,20 +48,20 @@ int ppc_cpu_gdb_read_register(CPUState *cs, uint8_t *mem_buf, int n)

      if (n < 32) {
          /* gprs */
-        return gdb_get_regl(mem_buf, env->gpr[n]);
+          return gdb_get_regl(mem_buf, end_swapl(env->gpr[n]));
      } else if (n < 64) {
          /* fprs */
          if (gdb_has_xml) {
              return 0;
          }
-        stfq_p(mem_buf, env->fpr[n-32]);
+        stfq_p(mem_buf, end_swapl(env->fpr[n-32]));
          return 8;
      } else {
          switch (n) {
          case 64:
-            return gdb_get_regl(mem_buf, env->nip);
+            return gdb_get_regl(mem_buf, end_swapl(env->nip));
          case 65:
-            return gdb_get_regl(mem_buf, env->msr);
+            return gdb_get_regl(mem_buf, end_swapl(env->msr));
          case 66:
              {
                  uint32_t cr = 0;
@@ -56,20 +69,20 @@ int ppc_cpu_gdb_read_register(CPUState *cs, uint8_t *mem_buf, int n)
                  for (i = 0; i < 8; i++) {
                      cr |= env->crf[i] << (32 - ((i + 1) * 4));
                  }
-                return gdb_get_reg32(mem_buf, cr);
+                return gdb_get_reg32(mem_buf, end_swap32(cr));
              }
          case 67:
-            return gdb_get_regl(mem_buf, env->lr);
+            return gdb_get_regl(mem_buf, end_swapl(env->lr));
          case 68:
-            return gdb_get_regl(mem_buf, env->ctr);
+            return gdb_get_regl(mem_buf, end_swapl(env->ctr));
          case 69:
-            return gdb_get_regl(mem_buf, env->xer);
+            return gdb_get_regl(mem_buf, end_swapl(env->xer));
          case 70:
              {
                  if (gdb_has_xml) {
                      return 0;
                  }
-                return gdb_get_reg32(mem_buf, env->fpscr);
+                return gdb_get_reg32(mem_buf, end_swap32(env->fpscr));
              }
          }
      }
@@ -83,47 +96,48 @@ int ppc_cpu_gdb_write_register(CPUState *cs, uint8_t *mem_buf, int n)

      if (n < 32) {
          /* gprs */
-        env->gpr[n] = ldtul_p(mem_buf);
+        env->gpr[n] = end_swapl(ldtul_p(mem_buf));
          return sizeof(target_ulong);
      } else if (n < 64) {
          /* fprs */
          if (gdb_has_xml) {
              return 0;
          }
-        env->fpr[n-32] = ldfq_p(mem_buf);
+        env->fpr[n-32] = end_swapl(ldfq_p(mem_buf));
          return 8;
      } else {
          switch (n) {
          case 64:
-            env->nip = ldtul_p(mem_buf);
+            env->nip = end_swapl(ldtul_p(mem_buf));
              return sizeof(target_ulong);
          case 65:
-            ppc_store_msr(env, ldtul_p(mem_buf));
+            ppc_store_msr(env, end_swapl(ldtul_p(mem_buf)));
              return sizeof(target_ulong);
          case 66:
              {
                  uint32_t cr = ldl_p(mem_buf);
                  int i;
                  for (i = 0; i < 8; i++) {
-                    env->crf[i] = (cr >> (32 - ((i + 1) * 4))) & 0xF;
+                    env->crf[i] = end_swap32((cr >> (32 -
+                                             ((i + 1) * 4))) & 0xF);
                  }
                  return 4;
              }
          case 67:
-            env->lr = ldtul_p(mem_buf);
+            env->lr = end_swapl(ldtul_p(mem_buf));
              return sizeof(target_ulong);
          case 68:
-            env->ctr = ldtul_p(mem_buf);
+            env->ctr = end_swapl(ldtul_p(mem_buf));
              return sizeof(target_ulong);
          case 69:
-            env->xer = ldtul_p(mem_buf);
+            env->xer = end_swapl(ldtul_p(mem_buf));
              return sizeof(target_ulong);
          case 70:
              /* fpscr */
              if (gdb_has_xml) {
                  return 0;
              }
-            store_fpscr(env, ldtul_p(mem_buf), 0xffffffff);
+            store_fpscr(env, end_swapl(ldtul_p(mem_buf)), 0xffffffff);
              return sizeof(target_ulong);
          }
      }
-- 1.8.3.1