Re: [PATCH v2] piix: fix regression during unplug in Xen HVM domUs

[Olaf Hering]

Resuming this old thread about an unfixed bug, which was introduced in qemu-4.2:

qemu ends up in piix_ide_reset from pci_unplug_disks.
This was not the case prior 4.2, the removed call to
qemu_register_reset(piix3_reset, d) in
ee358e919e385fdc79d59d0d47b4a81e349cd5c9 did apparently nothing.

In my debugging (with v8.0.0) it turned out the three pci_set_word
causes the domU to hang. In fact, it is just the last one:

   pci_set_byte(pci_conf + 0x20, 0x01);  /* BMIBA: 20-23h */

It changes the value from 0xc121 to 0x1.

The question is: what does this do in practice?

Starting with recent qemu (like 7.2), the domU sometimes proceeds with
these messages:

    [    1.631161] uhci_hcd 0000:00:01.2: host system error, PCI problems?
    [    1.634965] uhci_hcd 0000:00:01.2: host controller process error, 
something bad happened!
    [    1.634965] uhci_hcd 0000:00:01.2: host controller halted, very bad!
    [    1.634965] uhci_hcd 0000:00:01.2: HC died; cleaning up
    Loading basic drivers...[    2.398048] Disabling IRQ #23

Is anyone familiar enough with PIIX3 and knows how these devices are
interacting?

00:00.0 Host bridge: Intel Corporation 440FX - 82441FX PMC [Natoma] (rev 02)
00:01.0 ISA bridge: Intel Corporation 82371SB PIIX3 ISA [Natoma/Triton II]
00:01.1 IDE interface: Intel Corporation 82371SB PIIX3 IDE [Natoma/Triton II]
00:01.2 USB controller: Intel Corporation 82371SB PIIX3 USB [Natoma/Triton II] 
(rev 01)
00:01.3 Bridge: Intel Corporation 82371AB/EB/MB PIIX4 ACPI (rev 03)
00:02.0 Unassigned class [ff80]: XenSource, Inc. Xen Platform Device (rev 01)
00:03.0 VGA compatible controller: Cirrus Logic GD 5446


Thanks,
Olaf

On Thu, Mar 25, Paolo Bonzini wrote:

> On 25/03/21 12:12, Olaf Hering wrote:
> > Am Mon, 22 Mar 2021 18:09:17 -0400
> > schrieb John Snow <jsnow@redhat.com>:
> > 
> > > My understanding is that XEN has some extra disks that it unplugs when
> > > it later figures out it doesn't need them. How exactly this works is
> > > something I've not looked into too closely.
> > 
> > It has no extra disks, why would it?
> > 
> > I assume each virtualization variant has some sort of unplug if it has to 
> > support guests that lack PV/virtio/enlightened/whatever drivers.
> 
> No, it's Xen only and really should be legacy.  Ideally one would just have
> devices supported at all levels from firmware to kernel.
> 
> > > So if these IDE devices have been "unplugged" already, we avoid
> > > resetting them here. What about this reset causes the bug you describe
> > > in the commit message?
> > > 
> > > Does this reset now happen earlier/later as compared to what it did
> > > prior to ee358e91 ?
> > 
> > Prior this commit, piix_ide_reset was only called when the entire
> > emulated machine was reset. Like: never. With this commit,
> > piix_ide_reset will be called from pci_piix3_xen_ide_unplug. For some
> > reason it confuses the emulated USB hardware. Why it does confused
> > it, no idea.
> 
> > I wonder what the purpose of the qdev_reset_all() call really is. It
> > is 10 years old. It might be stale.
> 
> piix_ide_reset is only calling ide_bus_reset, and from there ide_reset and
> bmdma_reset.  All of these functions do just two things: reset internal
> registers and ensure pending I/O is completed or canceled.  The latter is
> indeed unnecessary; drain/flush/detach is already done before the call to
> qdev_reset_all.
> 
> But the fact that it breaks USB is weird.  That's the part that needs to be
> debugged, because changing IDE to unbreak USB needs an explanation even if
> it's the right thing to do.
> 
> If you don't want to debug it, removing the qdev_reset_all call might do the
> job; you'll have to see what the Xen maintainers think of it.  But if you
> don't debug the USB issue now, it will come back later almost surely.
> 
> Paolo

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