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

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Sat, 2024-10-05 at 12:51 -0700, Richard Henderson wrote:
> On 9/25/24 00:43, Ilya Leoshkevich wrote:
> > 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).
> 
> Of course it would, otherwise "exclusive" has no meaning.
> All other cpus are blocked in exclusive_idle().
> 
> Importantly, no cpus are blocked in syscalls, where the kernel can
> modify memory behind 
> gdbstub's back (e.g. read).  I think considering "in_syscall" to be
> "paused" a mistake.
> 
> 
> r~

How can we handle the long-running syscalls?
Just waiting sounds unsatisfying.
Sending a reserved host signal may alter the guest's behaviour if a
syscall like pause() is interrupted.
What do you think about SIGSTOP-ping the "in_syscall" threads?
A quick experiment shows that it should be completely invisible to the
guest - the following program continues to run after SIGSTOP/SIGCONT:

#include <sys/syscall.h>
#include <unistd.h>
int main(void) { syscall(__NR_pause); };