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Re: [PATCH] tcg: gdbstub: Fix single-step issue on arm target

From: Luc Michel
Subject: Re: [PATCH] tcg: gdbstub: Fix single-step issue on arm target
Date: Thu, 20 Feb 2020 22:24:37 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0


On 2/20/20 4:58 PM, Changbin Du wrote:
> Recently when debugging an arm32 system on qemu, I found sometimes the
> single-step command (stepi) is not working. This can be reproduced by
> below steps:
>  1) start qemu-system-arm -s -S .. and wait for gdb connection.
>  2) start gdb and connect to qemu. In my case, gdb gets a wrong value
>     (0x60) for PC.
>  3) After connected, type 'stepi' and expect it will stop at next ins.
> But, it has never stopped. This because:
>  1) We doesn't report ‘vContSupported’ feature to gdb explicitly and gdb
>     think we do not support it. In this case, gdb use a software breakpoint
>     to emulate single-step.
>  2) Since gdb gets a wrong initial value of PC, then gdb inserts a
>     breakpoint to wrong place (PC+4).
> Since we do support ‘vContSupported’ query command, so let's tell gdb that
> we support it.
> Before this change, gdb send below 'Z0' packet to implement single-step:
> gdb_handle_packet: Z0,4,4
> After this change, gdb send "vCont;s.." which is expected:
> gdb_handle_packet: vCont?
> put_packet: vCont;c;C;s;S
> gdb_handle_packet: vCont;s:p1.1;c:p1.-1
I'm curious, I never experienced this behaviour from GDB. What GDB and
QEMU versions are you using?

On my side (GDB 9.1), even without 'vContSupported+' in the 'qSupported'
answer, GDB sends a 'vCont?' packet on the first stepi:

0x00000000 in ?? ()
(gdb) si
Sending packet: $m0,4#fd...Ack
Packet received: 00000000
Sending packet: $vCont?#49...Ack
Packet received: vCont;c;C;s;S
Packet vCont (verbose-resume) is supported
Sending packet: $vCont;s:p1.1;c:p1.-1#f7...Ack
Packet received: T05thread:p01.01;

Your second issue (wrong PC value) should be investigated though. Does
it happen on QEMU vanilla? Do you have a way to reproduce this bug?

Anyway after re-reading the GDB remote protocol documentation, I think
your patch is right, the feature should be advertised.

However I think your commit message needs some modifications. This fix
is not specific to ARM or TCG, but to the gdbstub itself. You also
mention this bug you have with PC, which is not related to the bug you
are fixing here. Could you rewrite it in a more generic way? You simply
need to emphasis the effect of advertising the 'vContSupported+' feature
on GDB.



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