Re: MSI-X cleanup(?) issue with passthrough after domU restart

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Tue, Aug 26, 2025 at 04:47:50PM +0200, Roger Pau Monné wrote:
> On Tue, Aug 26, 2025 at 03:55:05PM +0200, Marek Marczykowski-Górecki wrote:
> > On Tue, Aug 26, 2025 at 12:57:50PM +0200, Marek Marczykowski-Górecki wrote:
> > > On Tue, Aug 26, 2025 at 10:28:56AM +0200, Roger Pau Monné wrote:
> > > > On Tue, Aug 26, 2025 at 08:16:56AM +0200, Jan Beulich wrote:
> > > > > On 26.08.2025 03:49, Marek Marczykowski-Górecki wrote:
> > > > > > Hi,
> > > > > > 
> > > > > > I'm hitting an MSI-X issue after rebooting the domU. The symptoms are
> > > > > > rather boring: on initial domU start the device (realtek eth card) works
> > > > > > fine, but after domU restart, the link doesn't come up (there is no
> > > > > > "Link is Up" message anymore). No errors from domU driver or Xen. I
> > > > > > tracked it down to MSI-X - if I force INTx (via pci=nomsi on domU
> > > > > > cmdline) it works fine. Convincing the driver to poll instead of waiting
> > > > > > for an interrupt also workarounds the issue.
> > > > > > 
> > > > > > I noticed also some interrupts are not cleaned up on restart. The list
> > > > > > of MSIs in 'Q' debug key output grows:
> > > > > > 
> > > > > >     (XEN) 0000:03:00.0 - d22 - node -1  - MSIs < 41 42 43 44 45 46 47 >
> > > > > >     restart sys-net domU
> > > > > >     (XEN) 0000:03:00.0 - d24 - node -1  - MSIs < 41 42 43 44 45 46 47 48 >
> > > > > >     restart sys-net domU
> > > > > >     (XEN) 0000:03:00.0 - d26 - node -1  - MSIs < 41 42 43 44 45 46 47 48 49 >
> > > > > > 
> > > > > > and 'M' output is:
> > > > > > 
> > > > > >     (XEN)  MSI-X   41 vec=b1 lowest  edge   assert  log lowest dest=00000001 mask=1/H /1
> > > > > >     (XEN)  MSI-X   42 vec=b9 lowest  edge   assert  log lowest dest=00000004 mask=1/HG/1
> > > > > >     (XEN)  MSI-X   43 vec=c1 lowest  edge   assert  log lowest dest=00000010 mask=1/HG/1
> > > > > >     (XEN)  MSI-X   44 vec=d9 lowest  edge   assert  log lowest dest=00000001 mask=1/HG/1
> > > > > >     (XEN)  MSI-X   45 vec=e1 lowest  edge   assert  log lowest dest=00000001 mask=1/HG/1
> > > > > >     (XEN)  MSI-X   46 vec=e9 lowest  edge   assert  log lowest dest=00000040 mask=1/HG/1
> > > > > >     (XEN)  MSI-X   47 vec=32 lowest  edge   assert  log lowest dest=00000004 mask=1/HG/1
> > > > > >     (XEN)  MSI-X   48 vec=3a lowest  edge   assert  log lowest dest=00000040 mask=1/HG/1
> > > > > >     (XEN)  MSI-X   49 vec=42 lowest  edge   assert  log lowest dest=00000010 mask=1/ G/1
> > > > > > 
> > > > > > And also, after starting and stopping the domU, `xl pci-assignable-remove 03:00.0`
> > > > > > makes pciback to complain:
> > > > > > 
> > > > > >     [ 1180.919874] pciback 0000:03:00.0: xen_pciback: MSI-X release failed (-16)
> > > > > > 
> > > > > > This is all running on Xen 4.19.3, but I don't see much changes in this
> > > > > > area since then.
> > > > > > 
> > > > > > Some more info collected at https://github.com/QubesOS/qubes-issues/issues/9335
> > > > > > 
> > > > > > My question is: what should be responsible for this cleanup on domain
> > > > > > destroy? Xen, or maybe device model (which is QEMU in stubdomain here)?
> > > > > 
> > > > > The expectation is that qemu invokes the necessary cleanup, but of course ...
> > > > > 
> > > > > > I see some cleanup (apparently not enough) happening via QEMU when the
> > > > > > domU driver is unloaded, but logically correct cleanup shouldn't depend
> > > > > > on correct domU operation...
> > > > > 
> > > > > ... Xen may not make itself dependent upon either DomU or QEMU.
> > > > 
> > > > AFAICT free_domain_pirqs() called by arch_domain_destroy() should take
> > > > care of unbinding and freeing pirqs (but obviously not in this case).
> > > > Can you repeat the test with a debug=y hypervisor and post the
> > > > resulting serial or dmesg here?  Some of the errors on those paths are
> > > > printed with dprintk() and won't be visible unless using a Xen debug
> > > > build.
> > > 
> > > Sure, will do.
> > 
> > Output collected during domU shutdown and subsequent startup (dom0 logs
> > to Xen console here too):
> > https://gist.github.com/marmarek/6dc3ac14d3ba840482e6361fcbd37c30
> > 
> > I don't see any errors there...
> 
> Hm, yes, I don't see any errors either.  Do you think you could
> instrument unmap_domain_pirq() and figure out whether it gets called,
> and if such call to unmap the PIRQ succeeds?

Sure, now that I know where to look at, I'll try to find out what goes
wrong.

> There's one silent error path in the return value of
> xsm_unmap_domain_irq(), but that shouldn't be taken since the call to
> unmap_domain_pirq() executed when destroying the domain is done with
> d->is_dying == true.
> 
> > As for the domU-triggered cleanup, I just checked - if I unload the
> > driver in domU before restarting, it works fine on subsequent startup.
> 
> Yeah, the unbind an unmap is then done by QEMU, and that seems to
> work fine.  What doesn't seem to work fine is the cleanup done as part
> of domain destroy.
> 
> Don't we have any Gitlab tests that do PCI passthrough?  Maybe those
> don't do any domU resets?

Yeah, it's a single boot and that's it. I can improve that (and double
check if MSI-X is covered too).

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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