target/i386/gdbstub: Fix a bug about order of FPU stack in 'g' packets.

Before this commit, when GDB attached an OS working on QEMU, order of FPU
stack registers printed by GDB command 'info float' was wrong. There was a
bug causing the problem in 'g' packets sent by QEMU to GDB. The packets have
values of registers of machine emulated by QEMU containing FPU stack
registers. There are 2 ways to specify a x87 FPU stack register. The first
is specifying by absolute indexed register names (R0, ..., R7). The second
is specifying by stack top relative indexed register names (ST0, ..., ST7).
Values of the FPU stack registers should be located in 'g' packet and be
ordered by the relative index. But QEMU had located these registers ordered
by the absolute index. After this commit, when QEMU reads registers to make
a 'g' packet, QEMU specifies FPU stack registers by the relative index.
Then, the registers are ordered correctly in the packet. As a result, GDB,
the packet receiver, can print FPU stack registers in the correct order.

Signed-off-by: TaiseiIto <taisei1212@outlook.jp>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <TY0PR0101MB4285923FBE9AD97CE832D95BA4E59@TY0PR0101MB4285.apcprd01.prod.exchangelabs.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/gdbstub.c

@@ -121,7 +121,9 @@ int x86_cpu_gdb_read_register(CPUState *cs, GByteArray *mem_buf, int n)
             return gdb_get_reg32(mem_buf, env->regs[gpr_map32[n]]);
         }
     } else if (n >= IDX_FP_REGS && n < IDX_FP_REGS + 8) {
-        floatx80 *fp = (floatx80 *) &env->fpregs[n - IDX_FP_REGS];
+        int st_index = n - IDX_FP_REGS;
+        int r_index = (st_index + env->fpstt) % 8;
+        floatx80 *fp = &env->fpregs[r_index].d;
         int len = gdb_get_reg64(mem_buf, cpu_to_le64(fp->low));
         len += gdb_get_reg16(mem_buf, cpu_to_le16(fp->high));
         return len;