How to handle Hotplug with UIO userspace driver

[Mandeep Sandhu]

Hi All,

We're working on a custom PCIe based hardware, which can be hotplugged into
the system.

We're using the UIO approach of having a minimal kernel driver which
basically handles the interrupt and provides a mmap of the device memory to
userspace and userspace driver which mmap's the provided memory and manages
the device.

We're using systemd and udev rules to spawn off a user process whenever the
hardware is plugged into the system.

Now my question is, how can we gracefully inform the userspace process if
someone pulls out the hardware (sitting on a PCI slot). Even with full
hotplug support, there can always be a case of "surprise removal", where
someone yanks the board without going through the whole "attention button"
routine.

While the kernel driver has no problem with hot-removal, I see that if I
"unregister" our UIO driver in response to the removal, then the mapping
for the hardware's physical memory would become "invalid" (done by UIO on
unregsiter). This could possibly cause the user-space process to crash due
to illegal memory access.

Is there a "standard" way to handle this scenario, i.e for hotpluggable
hardware using UIO?

Basically, we'd want to "unregister" the UIO device only after the
userspace process is informed of the removal so that it could unmap/close
the device and exit gracefully.

We could try doing it using some systemd trick, where we put a dependency
on one of the UIO created files (/dev/uioX), so that when the file is
destroyed, systemd could kill/shutdown the process. Although, there's no
guarantee that the userspace process won't try to access the mmaped space
during that interval, so this isn't a reliable solution either.

Any hints, thoughts?

Thanks.,
-mandeep
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