gdb for Riscv, single stepping issue

James Becker james.becker@crossfire-tech.com
Sun Jul 3 03:58:39 GMT 2022


Hi Pedro,

I did make the change and recompiled, but I have unfortunately lost my hardware program to my client for testing.

So it will be a week or two until I can test.

Thanks for your help.

Jim
________________________________
From: Pedro Alves <pedro@palves.net>
Sent: Thursday, June 30, 2022 4:40 AM
To: John Baldwin <jhb@FreeBSD.org>; James Becker <james.becker@crossfire-tech.com>; gdb@sourceware.org <gdb@sourceware.org>
Subject: Re: gdb for Riscv, single stepping issue

Hi!

Jim, could you check whether the patch below addresses the issue for you?
I imagine it does, but it'd be good to hear just in case we're missing some
more target_read_memory calls somewhere or some such.

Pedro Alves

On 2022-06-29 18:54, John Baldwin wrote:
> On 6/29/22 2:34 AM, Pedro Alves wrote:

>> OTOH, riscv_insn::fetch_instruction uses target_read_memory to read 2 bytes,
>> so I wonder whether this is the access in question:
>>
>>    /* All insns are at least 16 bits.  */
>>    status = target_read_memory (addr, buf, 2);
>>    if (status)
>>      memory_error (TARGET_XFER_E_IO, addr);
>>
>> Why is this using target_read_memory instead of target_read_code, though?
>> If it did that, then the code cache would be involved here too, papering over
>> the issue, presumably.
>>
> Yes, I had reworked my e-mail part way through and I really meant to talk about
> this code, not the breakpoint code.  I agree that target_read_code is probably
> more correct in fetch_instruction and would in this case hide the odd behavior
> of the stub:
>
> diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
> index 69f2123dcdb..09b2599e958 100644
> --- a/gdb/riscv-tdep.c
> +++ b/gdb/riscv-tdep.c
> @@ -1661,7 +1661,7 @@ riscv_insn::fetch_instruction (struct gdbarch *gdbarch,
>    int instlen, status;
>
>    /* All insns are at least 16 bits.  */
> -  status = target_read_memory (addr, buf, 2);
> +  status = target_read_code (addr, buf, 2);
>    if (status)
>      memory_error (TARGET_XFER_E_IO, addr);
>
> @@ -1672,7 +1672,7 @@ riscv_insn::fetch_instruction (struct gdbarch *gdbarch,
>
>    if (instlen > 2)
>      {
> -      status = target_read_memory (addr + 2, buf + 2, instlen - 2);
> +      status = target_read_code (addr + 2, buf + 2, instlen - 2);
>        if (status)
>      memory_error (TARGET_XFER_E_IO, addr + 2);
>      }
>
>



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