Re: [PATCH 1/2] gdbstub: Set current_cpu for memory read write

["Alex Bennée" <alex.bennee@linaro.org>]

Bin Meng <bmeng.cn@gmail.com> writes:

> On Sat, Apr 2, 2022 at 7:20 PM Bin Meng <bmeng.cn@gmail.com> wrote:
>>
>> On Tue, Mar 29, 2022 at 12:43 PM Bin Meng <bmeng.cn@gmail.com> wrote:
>> >
>> > On Mon, Mar 28, 2022 at 5:10 PM Peter Maydell <peter.maydell@linaro.org> wrote:
>> > >
>> > > On Mon, 28 Mar 2022 at 03:10, Bin Meng <bmeng.cn@gmail.com> wrote:
>> > > > IMHO it's too bad to just ignore this bug forever.
>> > > >
>> > > > This is a valid use case. It's not about whether we intentionally want
>> > > > to inspect the GIC register value from gdb. The case is that when
>> > > > single stepping the source codes it triggers the core dump for no
>> > > > reason if the instructions involved contain load/store to any of the
>> > > > GIC registers.
>> > >
>> > > Huh? Single-stepping the instruction should execute it inside
>> > > QEMU, which will do the load in the usual way. That should not
>> > > be going via gdbstub reads and writes.
>> >
>> > Yes, single-stepping the instruction is executed in the vCPU context,
>> > but a gdb client sends additional commands, more than just telling
>> > QEMU to execute a single instruction.
>> >
>> > For example, the following is the sequence a gdb client sent when doing a "si":
>> >
>> > gdbstub_io_command Received: Z0,100000,4
>> > gdbstub_io_reply Sent: OK
>> > gdbstub_io_got_ack Got ACK
>> > gdbstub_io_command Received: m18c430,4
>> > gdbstub_io_reply Sent: ff430091
>> > gdbstub_io_got_ack Got ACK
>> > gdbstub_io_command Received: vCont;s:p1.1;c:p1.-1
>> > gdbstub_op_stepping Stepping CPU 0
>> > gdbstub_op_continue_cpu Continuing CPU 1
>> > gdbstub_op_continue_cpu Continuing CPU 2
>> > gdbstub_op_continue_cpu Continuing CPU 3
>> > gdbstub_hit_break RUN_STATE_DEBUG
>> > gdbstub_io_reply Sent: T05thread:p01.01;
>> > gdbstub_io_got_ack Got ACK
>> > gdbstub_io_command Received: g
>> > gdbstub_io_reply Sent:
>> > 3848ed0000000000f08fa610000000000300000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000010000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001f90000000030a5ec000000000034c4180000000000c9030000
>> > gdbstub_io_got_ack Got ACK
>> > gdbstub_io_command Received: m18c434,4
>> > gdbstub_io_reply Sent: 00e004d1
>> > gdbstub_io_got_ack Got ACK
>> > gdbstub_io_command Received: m18c430,4
>> > gdbstub_io_reply Sent: ff430091
>> > gdbstub_io_got_ack Got ACK
>> > gdbstub_io_command Received: m18c434,4
>> > gdbstub_io_reply Sent: 00e004d1
>> > gdbstub_io_got_ack Got ACK
>> > gdbstub_io_command Received: m18c400,40
>> > gdbstub_io_reply Sent:
>> > ff4300d1e00300f980370058000040f900a00191000040f900b00091000040f900e004911e7800f9fe0340f91e0000f9ff43009100e004d174390094bb390094
>> > gdbstub_io_got_ack Got ACK
>> > gdbstub_io_command Received: mf9010000,4
>> >
>> > Here "mf9010000,4" triggers the bug where 0xf9010000 is the GIC register.
>> >
>> > This is not something QEMU can ignore or control. The logic is inside
>> > the gdb client.
>> >
>>
>> Ping for this series?
>>
>
> Ping?

Re-reading the thread we seem to have two problems:

  - gdbstub is not explicitly passing the explicit CPU for its access
  - some devices use current_cpu to work out what AS they should be
    working in

But I've already said just fudging current_cpu isn't the correct
approach.

-- 
Alex Bennée