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

From: Changbin Du
Subject: Re: [PATCH] tcg: gdbstub: Fix single-step issue on arm target
Date: Fri, 21 Feb 2020 08:07:11 +0800

On Thu, Feb 20, 2020 at 10:24:37PM +0100, Luc Michel wrote:
> Hi,
> 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?
For QEMU, it's built from mainline.
For GDB, I have tried 8.1 and latest 9.1.

> 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;
hmm, On my side, this is 100% reproducable on arm32, but aarch64 doesn't. I
think the GDB has different assumptions for different arch.

> 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?
This is also 100% reproducable for my tested elf guest. But so sorry that I
can't share it. Probablly I will check this issue some days later.

> 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.

> Thanks.
> -- 
> Luc

Changbin Du

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