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Re: [PATCH 11/11] New target_ops hook to_can_do_single_step
- From: Pedro Alves <palves at redhat dot com>
- To: Yao Qi <qiyaoltc at gmail dot com>, gdb-patches at sourceware dot org
- Date: Wed, 01 Jul 2015 17:42:58 +0100
- Subject: Re: [PATCH 11/11] New target_ops hook to_can_do_single_step
- Authentication-results: sourceware.org; auth=none
- References: <1435759111-22856-1-git-send-email-yao dot qi at linaro dot org> <1435759111-22856-12-git-send-email-yao dot qi at linaro dot org>
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