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

[Richard Henderson]

On 12/8/22 04:30, TaiseiIto wrote:
> 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>
> ---
>   target/i386/gdbstub.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/target/i386/gdbstub.c b/target/i386/gdbstub.c
> index c3a2cf6f28..6109ad166d 100644
> --- a/target/i386/gdbstub.c
> +++ b/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 = (floatx80 *) &env->fpregs[r_index];

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

Note that the cast can be dropped by taking the address of the ".d" union 
member.


r~