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

[Daniel P . Berrangé]

On Thu, Nov 16, 2023 at 04:47:48PM +0530, Ani Sinha wrote:
> 
> 
> > On 16-Nov-2023, at 2:25 PM, Daniel P. Berrangé <berrange@redhat.com> wrote:
> > 
> > On Thu, Nov 16, 2023 at 05:14:43PM +1000, Nicholas Piggin wrote:
> >> On Thu Nov 16, 2023 at 1:55 PM AEST, Ani Sinha wrote:
> >>> 
> >>> 
> >>>> On 16-Nov-2023, at 6:45 AM, Nicholas Piggin <npiggin@gmail.com> wrote:
> >>>> 
> >>>> 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:
> >>>>> 
> >>>>> 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) ? 
> >>> 
> >>> I think what he is saying is to add a new property for QEMUMachine() with 
> >>> which the test can explicitly tell the machine init code that it is going 
> >>> to drain the console logs. By default it can be false. When tests use 
> >>> console_drainer, they can set the property to true and inspect the early 
> >>> console logs after draining it. 
> >> 
> >> Hmm... well we do have QEMUMachine.set_console already. Is this enough?
> >> If the test case is not going to drain or interact with the console
> >> then it could set it to false. Or am I missing something?
> > 
> > Yeah, set_console is enough - i missed that that exists.
> > 
> > Thus problem is more specific. It hits when a test calls
> > set_console(True), but then fails to read from the console.
> 
> So then it is a test issue in that the test requests console to be enabled 
> but does not look at the console o/p.

Yes, any test cases affected by this are broken already, and need to drain
the console. We might easily miss if we only test with x86_64 QEMU and not
other arches, as eg ppc firmware appears more chatty on serial port than
seabios

With regards,
Daniel
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