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. > sure. > Thanks. > > -- > Luc -- Cheers, Changbin Du
