[Bug 1893778] Comment bridged from LTC Bugzilla

------- Comment From heinz-werner_seeck@de.ibm.com 2020-10-20 08:40 EDT-------
IBM Bugzilla status-> closed, Fix Released with all requested Distros

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https://bugs.launchpad.net/bugs/1893778

Title:
[UBUNTU 20.04] zPCI device hot-plug during boot may result in unusable
device

Status in Ubuntu on IBM z Systems:
Fix Released
Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Focal:
Fix Released
Status in linux source package in Groovy:
Fix Released

Bug description:
SRU Justification:
==================

[Impact]

* If a PCI device (incl. virtio-pci) is hot-plugged during boot-up on
s390x, it can be detected as an entry in CLP List PCI functions and
via the hot-plug event.

* (This is basically equivalent to boot time probing on other
architectures.)

* In such a case the hot-plug event will be stale, but Linux still
tries to add and enable the device which leads to:

* a) a duplicate entry in zPCI internal device list

* b) an attempt to enable the device with a stale function handle

* In case b) the device will be placed in error state which makes it
unusable.

[Fix]

* b76fee1bc56c31a9d2a49592810eba30cc06d61a b76fee1bc56c "s390/pci:
ignore stale configuration request event"

[Test Case]

* Setup an Ubuntu Server 20.04 (focal) Linux operating system on an
IBM Z or LinuxONE III LPAR.

* It's now easiest to test on KVM using virtio-pci (on s390x).

* Start a test virtual machine: sudo virsh start <test-guest>

* Attach and hotplug a virtio-pci device: sudo virsh attach-device
<test-guest> hotplug_pci_block.xml

* Where hotplug_pci_block.xml looks like:
<disk device="disk" type="file">
<driver name="qemu" type="raw" />
<address type="pci">
<zpci fid="4660" uid="4660" />
</address>
<source file="testdisk.img" />
<target bus="virtio" dev="vdt" />
</disk>

[Regression Potential]

* The regression risk is moderate, since the modification is very
limited and therefore manageable (additional if statement - two lines
of code) and easily testable on KVM using virtio-pci.

* The changes are in the zPCI event code, so in worst-case it can
happen that the event handling get harmed which may break zPCI
entirely, affecting all PCI devices incl. virtio-pci (on s390x).

* A bug in PCI 'availability' handling also just lead to wrong states
of PCI devices which make them unavailable, hence unusable.

* Notice that zPCI is the s390x-specific PCI implementation,
modifications here do not affect any other architecture.

* And zPCI devices are less wide-spread compared to ccw devices on
s390x.

* On top a test kernel was build and made available for further
testing atesting can be easily done with virtio-pci on KVM.

[Other]

* The fix/patch got upstream accepted with kernel v5.9-rc2.

* But it landed already in groovy's proposed kernel 5.8
(Ubuntu-5.8.0-18.19), due to 'Groovy update: v5.8.4 upstream stable
release' that is handled in LP 1893048.

* Hence this fix/patch need to be applied to focal only.

__________

When a PCI device (including virtio-pci for which this is easiest to test)
is hot-plugged while Linux is still booting, it can be detected as
an entry in CLP List PCI Functions (basically equivalent to boot time probing
on other architectures) and with the hot-plug event.
In this case the hot-plug event will be stale but Linux still
tried to add and enable the device leading

a) to a duplicate entry in zPCI internal device list
b) an attempt to enable the device witha stale function handle

Part b) would lead to the device being place in the error state
and make it unusable.

This can most easily be reproduced using KVM and doing

# sudo virsh start myguest && sudo virsh attach-device myguest
hotplug_pci_block.xml

Where hotplug_pci_block.xml looks like the following:

<disk device="disk" type="file">
        <driver name="qemu" type="raw" />
        <address type="pci">
                <zpci fid="4660" uid="4660" />
        </address>
        <source file="testdisk.img" />
        <target bus="virtio" dev="vdt" />
</disk>

The problem is fixed with the 3-line upstream commit

b76fee1bc56c31a9d2a49592810eba30cc06d61a s390/pci: ignore stale
configuration request event

I also confirmed that as of the focal tag Ubuntu-5.4.0-46.50 this
cherry-picks cleanly.

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