Only respond to Attention Button if device is already plugged in

[cperciva]

Summary

Currently if PCIEM_SLOT_STA_ABP and PCIEM_SLOT_STA_PDC are asserted simultaneously, FreeBSD sets a 5 second "hardware going away" timer and then processes the "presence detect" change. In the (physically challenging) case that someone presses the "attention button" and inserts a new PCIe device at exactly the same moment, this results in FreeBSD recognizing that the device is present, attaching it, and then detaching it 5 seconds later.

On EC2 "bare metal" hardware this is the precise sequence of events which takes place when a new EBS volume is attached; virtual machines have no difficulty effecting physically implausible simultaneity.

This patch changes the handling of PCIEM_SLOT_STA_ABP to only detach a device (or cancel the detach of a device) if the presence of a device was detected *before* the interrupt.

Test Plan

Works in EC2. Not sure if it's possible to test this fix anywhere else.

[jhb]

sys/dev/pci/pci_pci.c

1187 ↗

(On Diff #58184)

Maybe add a comment here, something like:

/* Ignore attention button presses if no device was previously present. */

A valid test even on real hardware would be if someone pressed the attention button when the slot was empty. We would currently queue a spurious timer instead of ignoring the button press. I think we should perhaps push the PDS check down into the "arming case" though so that we always honor a cancel button press.

[cperciva]

Moved test to the !PCIB_DETACH_PENDING case and added an explanatory comment.

[cperciva]

sys/dev/pci/pci_pci.c

1187 ↗

(On Diff #58184)

Good points, is this revised patch ok?

[jhb]

sys/dev/pci/pci_pci.c

1194 ↗

(On Diff #58207)

Maybe add 'device was previously present' since we are using 'old_slot_sta', but looks good to me.

[mqudsi_neosmart.net]

This works for VMware ESXi as well, where network card add/remove is directly mapped to PCIe hotplug.

It's a pretty clean (and reasonable) fix; the only other obvious workaround would be to cancel extant device removal timers upon insert (if it were desirable to preserve the "attention button" signal for some odd reason), but I see no reason to complicate things and go that route.