Re: handle_pio looping during domain shutdown, with qemu 4.2.0 in stubdom

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Fri, Jun 05, 2020 at 11:38:17AM +0200, Jan Beulich wrote:
> On 04.06.2020 03:46, Marek Marczykowski-Górecki wrote:
> > Hi,
> > 
> > (continuation of a thread from #xendevel)
> > 
> > During system shutdown quite often I hit infinite stream of errors like
> > this:
> > 
> >     (XEN) d3v0 Weird PIO status 1, port 0xb004 read 0xffff
> >     (XEN) domain_crash called from io.c:178
> > 
> > This is all running on Xen 4.13.0 (I think I've got this with 4.13.1
> > too), nested within KVM. The KVM part means everything is very slow, so
> > various race conditions are much more likely to happen.
> > 
> > It started happening not long ago, and I'm pretty sure it's about
> > updating to qemu 4.2.0 (in linux stubdom), previous one was 3.0.0.
> > 
> > Thanks to Andrew and Roger, I've managed to collect more info.
> > 
> > Context:
> >     dom0: pv
> >     dom1: hvm
> >     dom2: stubdom for dom1
> >     dom3: hvm
> >     dom4: stubdom for dom3
> >     dom5: pvh
> >     dom6: pvh
> > 
> > It starts I think ok:
> > 
> >     (XEN) hvm.c:1620:d6v0 All CPUs offline -- powering off.
> >     (XEN) d3v0 handle_pio port 0xb004 read 0x0000
> >     (XEN) d3v0 handle_pio port 0xb004 read 0x0000
> >     (XEN) d3v0 handle_pio port 0xb004 write 0x0001
> >     (XEN) d3v0 handle_pio port 0xb004 write 0x2001
> >     (XEN) d4v0 XEN_DMOP_remote_shutdown domain 3 reason 0
> 
> I can't seem to be able to spot the call site of this, in any of
> qemu, libxl, or libxc. I'm in particular curious as to the further
> actions taken on the domain after this was invoked: Do any ioreq
> servers get unregistered immediately (which I think would be a
> problem)?

It is here:
https://github.com/qemu/qemu/blob/master/hw/i386/xen/xen-hvm.c#L1539

I think it's called from cpu_handle_ioreq(), and I think the request
state is set to STATE_IORESP_READY before exiting (unless there is some
exit() hidden in another function used there).

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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