Re: [PATCH 11/11] New target_ops hook to_can_do_single_step

[Pedro Alves <palves@redhat.com>]

On 07/01/2015 02:58 PM, Yao Qi wrote:
> Nowadays, GDB only knows whether architecture supports hardware single
> step or software single step (through gdbarch hook software_single_step),
> and for a given instruction or instruction sequence, GDB knows how to
> do single step (hardware or software).  However, GDB doesn't know whether
> the target supports hardware single step.  It is possible that the
> architecture doesn't support hardware single step, such as arm, but
> the target supports, such as simulator.  This was discussed in this
> thread https://www.sourceware.org/ml/gdb/2009-12/msg00033.html before.
> 
> I encounter this problem for aarch64 multi-arch support.  When aarch64
> debugs arm program, gdbarch is arm, so software single step is still
> used.  However, the underneath linux kernel does support hardware
> single step, so IWBN to use it.
> 
> This patch is to add a new target_ops hook to_can_do_single_step, and
> only use it in arm_linux_software_single_step to decide whether or not
> to use hardware single step.  On the remote target, if the target
> supports s and S actions in the vCont? reply, then target can do single
> step.  On the native aarch64 linux target, 1 is returned.  On other
> targets, -1 is returned.

Yeah, I've wanted to do this before too.

But my issue with it is that this breaks gdb/gdbserver compatibility.

Old GDB has:

      /* If s, S, c, and C are not all supported, we can't use vCont.  Clearing
         BUF will make packet_ok disable the packet.  */
      if (!support_s || !support_S || !support_c || !support_C)
	buf[0] = 0;

Which means that new x86-86 gdbserver with old gdb will just
stop using vCont after this change.

And old arm gdbserver will still claim support for vCont;s packets,
which means that new gdb with old gdbserver will be broken.

I think this needs to be addressed somehow.

Thanks,
Pedro Alves