Re: [PATCH] tests/avocado/reverse_debugging: Disable the ppc64 tests by default

[Nicholas Piggin]

On Thu Nov 16, 2023 at 3:22 AM AEST, Daniel P. Berrangé wrote:
> On Wed, Nov 15, 2023 at 01:14:53PM +0000, Daniel P. Berrangé wrote:
> > On Wed, Nov 15, 2023 at 07:23:01AM +0100, Thomas Huth wrote:
> > > On 15/11/2023 02.15, Nicholas Piggin wrote:
> > > > On Wed Nov 15, 2023 at 4:29 AM AEST, Thomas Huth wrote:
> > > > > On 14/11/2023 17.37, Philippe Mathieu-Daudé wrote:
> > > > > > On 14/11/23 17:31, Thomas Huth wrote:
> > > > > > > The tests seem currently to be broken. Disable them by default
> > > > > > > until someone fixes them.
> > > > > > > 
> > > > > > > Signed-off-by: Thomas Huth <thuth@redhat.com>
> > > > > > > ---
> > > > > > >    tests/avocado/reverse_debugging.py | 7 ++++---
> > > > > > >    1 file changed, 4 insertions(+), 3 deletions(-)
> > > > > > 
> > > > > > Similarly, I suspect 
> > > > > > https://gitlab.com/qemu-project/qemu/-/issues/1961
> > > > > > which has a fix ready:
> > > > > > https://lore.kernel.org/qemu-devel/20231110170831.185001-1-richard.henderson@linaro.org/
> > > > > > 
> > > > > > Maybe wait the fix gets in first?
> > > > > 
> > > > > No, I applied Richard's patch, but the problem persists. Does this 
> > > > > test
> > > > > still work for you?
> > > > 
> > > > I bisected it to 1d4796cd008373 ("python/machine: use socketpair() for
> > > > console connections"),
> > > 
> > > Maybe John (who wrote that commit) can help?
> > 
> > I find it hard to believe this commit is a direct root cause of the
> > problem since all it does is change the QEMU startup sequence so that
> > instead of QEMU listening for a monitor connection, it is given a
> > pre-opened monitor connection.
> > 
> > At the very most that should affect the startup timing a little.
> > 
> > I notice all the reverse debugging tests have a skip on gitlab
> > with a comment:
> > 
> >     # unidentified gitlab timeout problem
> > 
> > this makes be suspicious that John's patch has merely made this
> > (henceforth undiagnosed) timeout more likely to ocurr.
>
> After an absolutely horrendous hours long debugging session I think
> I figured out the problem. The QEMU process is blocking in
>
>     qemu_chr_write_buffer
>
> spinning in the loop on EAGAIN.

Great work.

Why does this make the gdb socket give an empty response? Something
just times out?

>
> The Python  Machine() class has passed one of a pre-created socketpair
> FDs for the serial port chardev. The guest is trying to write to this
> and blocking.  Nothing in the Machine() class is reading from the
> other end of the serial port console.
>
>
> Before John's change, the serial port uses a chardev in server mode
> and crucially  'wait=off', and the Machine() class never opened the
> console socket unless the test case wanted to read from it.
>
> IOW, QEMU had a background job setting there waiting for a connection
> that would never come.
>
> As a result when QEMU started executing the guest, all the serial port
> writes get sent into to the void.
>
>
> So John's patch has had a semantic change in behaviour, because the
> console socket is permanently open, and thus socket buffers are liable
> to fill up.
>
> As a demo I increased the socket buffers to 1MB and everything then
> succeeded.
>
> @@ -357,6 +360,10 @@ def _pre_launch(self) -> None:
>  
>          if self._console_set:
>              self._cons_sock_pair = socket.socketpair()
> +            self._cons_sock_pair[0].setsockopt(socket.SOL_SOCKET, 
> socket.SO_SNDBUF, 1024*1024);
> +            self._cons_sock_pair[0].setsockopt(socket.SOL_SOCKET, 
> socket.SO_RCVBUF, 1024*1024);
> +            self._cons_sock_pair[1].setsockopt(socket.SOL_SOCKET, 
> socket.SO_SNDBUF, 1024*1024);
> +            self._cons_sock_pair[1].setsockopt(socket.SOL_SOCKET, 
> socket.SO_RCVBUF, 1024*1024);
>              os.set_inheritable(self._cons_sock_pair[0].fileno(), True)
>  
>          # NOTE: Make sure any opened resources are *definitely* freed in

So perhaps ppc64 fails just because it prints more to the console in early
boot than other targets?

> The Machine class doesn't know if anything will ever use the console,
> so as is the change is unsafe.
>
> The original goal of John's change was to guarantee we capture early
> boot messages as some test need that.  
>
> I think we need to be able to have a flag to say whether the caller needs
> an "early console" facility, and only use the pre-opened FD passing for
> that case. Tests we need early console will have to ask for that guarantee
> explicitly.

The below patch makes this test work. Maybe as a quick fix it is
better than disabling the test.

I guess we still have a problem if a test invokes vm.launch()
directly without subsequently waiting for a console pattern or
doing something with the console as you say. Your suggesstion is
add something like vm.launch(console=True) ? 

Thanks,
Nick
---

diff --git a/tests/avocado/reverse_debugging.py 
b/tests/avocado/reverse_debugging.py
index fc47874eda..128d85bc0e 100644
--- a/tests/avocado/reverse_debugging.py
+++ b/tests/avocado/reverse_debugging.py
@@ -12,6 +12,7 @@
 
 from avocado import skipIf
 from avocado_qemu import BUILD_DIR
+from avocado.utils import datadrainer
 from avocado.utils import gdb
 from avocado.utils import process
 from avocado.utils.network.ports import find_free_port
@@ -52,6 +53,10 @@ def run_vm(self, record, shift, args, replay_path, 
image_path, port):
         if args:
             vm.add_args(*args)
         vm.launch()
+        console_drainer = datadrainer.LineLogger(vm.console_socket.fileno(),
+                                    logger=self.log.getChild('console'),
+                                    stop_check=(lambda : not vm.is_running()))
+        console_drainer.start()
         return vm
 
     @staticmethod