Comment 0 for bug 1903682

Background:

When handling multifunction devices in zPCI we take the
UID of the PCI function with function number 0
(that always exists according to the PCI spec)
as domain number.
Therefore when hot plugging functions with function
number larger than 0 before function 0, we need
to hold these in standby before creating the
domain and bus.

This has been tested during feature development
using a patched QEMU and with DPM but never in Classic
Mode.

Reproduction:

This issue was introduced with the Topology aware PCI
Enumeration code so test with a Linux supporting
that feature. E.g. Upstream, Devel Driver etc.

On a Classic Mode machine with a multi-function device,
hot plug ("Reassign I/O Path") only the FID of the
second port to the LPAR.

Symptom:

After this any additional hotplug and even just
deconfiguring a PCI device will hang. A hotplug
makes the entire Linux instance unresponsive.

Analysis:

The problem occurs in Classic Mode but not with
previous testing as the LPAR hypervisor does
hot plug/Reassign I/O Path as a two step process:

1. zPCI event with PEC 0x0302 to plug the zPCI function in Standby
2. zPCI event with PEC 0x0301 to configure the zPCI function

For the first event we create the zdev in clp_add_pci_device()
in Standby which is all fine so far.
The problem then occurs in step 2 as we then find
the existing zdev and try to configure it.
This however does not work as the PCI bus
is not yet created (as we still don't know the UID of
function 0 that will become its domain).
The bus pointer zdev->zbus->bus pointer is thus still
NULL but will be accessed by common code which
inevitably results in disaster including
the above mentioned hang and (possibly) the below
RCU stall:

[ 689.724703] rcu: INFO: rcu_sched self-detected stall on CPU
[ 689.724712] rcu: 16-....: (42004 ticks this GP) idle=6ee/1/0x4000000000000002 softirq=1234/1234 fqs=14001
[ 689.724742] (t=42006 jiffies g=89 q=3770)
[ 689.724743] Task dump for CPU 16:
[ 689.724745] task:kmcheck state:R running task stack: 0 pid: 205 ppid: 2 flags:0x00000004
[ 689.724747] Call Trace:
[ 689.724757] [<0000000ccde0b5c4>] show_stack+0x8c/0xd8
[ 689.724762] [<0000000ccd0dabc4>] sched_show_task.part.0+0xe4/0x110
[ 689.724764] [<0000000ccde0ea5e>] rcu_dump_cpu_stacks+0xde/0x120
[ 689.724767] [<0000000ccd1465c6>] print_cpu_stall+0x266/0x330
[ 689.724768] [<0000000ccd14a428>] rcu_sched_clock_irq+0x618/0x670
[ 689.724771] [<0000000ccd15cd7a>] update_process_times+0xba/0xf0
[ 689.724775] [<0000000ccd1766fa>] tick_sched_timer+0x9a/0x220
[ 689.724777] [<0000000ccd15d962>] __hrtimer_run_queues+0x182/0x3a0
[ 689.724779] [<0000000ccd1602f8>] hrtimer_interrupt+0x138/0x450
[ 689.724782] [<0000000ccd0451c0>] do_IRQ+0x90/0xa0
[ 689.724784] [<0000000ccde2be96>] ext_int_handler+0x17e/0x184
[ 689.724790] [<0000000ccd9f373e>] pci_get_slot+0x5e/0xa0
[ 689.724794] [<0000000ccd9dc182>] pci_scan_single_device+0x32/0x2a0
[ 689.724797] [<0000000ccd0868f2>] __zpci_event_availability+0x192/0x360
[ 689.724800] [<0000000ccdd40c16>] chsc_process_crw+0x2e6/0x300
[ 689.724802] [<0000000ccdd4b088>] crw_collect_info+0x2b8/0x320
[ 689.724804] [<0000000ccd0caf3a>] kthread+0x14a/0x170
[ 689.724805] [<0000000ccde2b814>] ret_from_fork+0x24/0x2c

The fix is very simple, we check zdev->zbus->bus
for being NULL and in that case bail from the
case 0x0301 before calling the PCI common code
pci_scan_single_device() with the NULL pointer.

The only subtlety is that we still need to
do the zpci_enable_device() because the
code in arch/s390/pci/pci_bus.c assumes
that it can immediately do a scan of
all devfn != 0 PCI functions once
PCI function 0 is found.

It thereby mimics what happens
when we only find the FID for a function with
devfn != 0 in the CLP List PCI Functions.

This is implemented in the following upstream
commit:

0b2ca2c7d0c9e2731d01b6c862375d44a7e13923 s390/pci: fix hot-plug of PCI function missing bus

It is included in v5.10-rc3 and has been tagged for
stable > v5.8 i.e. all upstream versions with
the PCI enumeration changes.
Also it carries the appropriate Fixes tag.

I have verified that it cherry-picks cleanly
on current focal master-next and expect
it to cleanly cherry-pick on newer Ubuntu
Kernels too.