Re: [PATCH 00/18] Stop all qemu-cpu threads on a breakpoint

[Ilya Leoshkevich]

On Tue, 2024-09-24 at 13:46 +0200, Richard Henderson wrote:
> On 9/23/24 18:12, Ilya Leoshkevich wrote:
> > Hi,
> > 
> > On reporting a breakpoint in a non-non-stop mode, GDB remotes must
> > stop
> > all threads. Currently qemu-user doesn't do that, breaking the
> > debugging session for at least two reasons: concurrent access to
> > the
> > GDB socket, and an assertion within GDB [1].
> > 
> > This series fixes this by importing pause_all_vcpus() from qemu-
> > system.
> > This in turn requires introducing BQL and a few stubs to qemu-user.
> 
> I would have expected you to reuse (some portion of) start_exclusive,
> which is already 
> part of qemu-user.  Is there a reason you chose a solution which
> requires...
> 
> >    replay: Add replay_mutex_{lock,unlock}() stubs for qemu-user
> >    qemu-timer: Provide qemu_clock_enable() stub for qemu-user
> >    cpu: Use BQL in qemu-user
> 
> all sorts of other infrastructure?
> 
> 
> r~

I don't think start_exclusive() would protect the gdb socket from
concurrent accesses (e.g., if two threads are simultaneously stopped).

I have a patch [1] that introduces a big gdbstub lock for that, but it
looks more complex than just extending BQL to qemu-user. Also, the
BQL-based pause/resume code already works for the system mode and is
well tested.

[1]
https://gitlab.com/iii-i/qemu/-/commit/0944716218820f8bdfdcf80acc6c39a48b91670c