Re: [PATCH] pciehp: fast unplug for virtual machines

Krzysztof Wilczyński
2021-11-12T00:00:54Z

Hi Gerd,

> The PCIe specification asks the OS to wait five seconds after the
> attention button has been pressed before actually un-plugging the
> device. This gives the operator the chance to cancel the operation
> by pressing the attention button again within those five seconds.
>
> For physical hardware this makes sense. Picking the wrong button
> by accident can easily happen and it can be corrected that way.
>
> For virtual hardware the benefits are questionable. Typically
> users find the five second delay annoying.
>
> This patch adds the fast_virtual_unplug module parameter to the
> pciehp driver. When enabled (which is the default) the linux
> kernel will simply skip the delay for virtual pcie ports, which
> reduces the total time for the unplug operation from 6-7 seconds
> to 1-2 seconds.
>
> Virtual pcie ports are identified by PCI ID. For now the qemu
> pcie root ports are detected, other hardware can be added easily.

Bjorn asked to correct the PCIe references, whereas I would ask you to
change "linux" to "Linux", "qemu" to "QEMU", and generally capitalise
things where and as needed throughout.

> * @request_result: result of last user request submitted to the IRQ thread
> * @requester: wait queue to wake up on completion of user request,
> * used for synchronous slot enable/disable request via sysfs
> + * @is_virtual: virtual machine pcie port.

It would be "PCIe" here too for consistency.

> + if (dev->port->vendor == PCI_VENDOR_ID_REDHAT &&
> + dev->port->device == 0x000c)
> + /* qemu pcie root port */
> + ctrl->is_virtual = true;

I would personally move the comment above the if-statement as it looks
a little awkward added like that. You could also add a little bit more
detail why this is set for QEMU and what it's needed, etc. Also, if you
don't mind, let's change it to "QEMU" in the comment for consistency.

> - schedule_delayed_work(&ctrl->button_work, 5 * HZ);
> + if (ctrl->is_virtual && fast_virtual_unplug) {
> + schedule_delayed_work(&ctrl->button_work, 1);
> + } else {
> + schedule_delayed_work(&ctrl->button_work, 5 * HZ);
> + }

The brackets are fine but technically not needed above as per the kernel
coding style.

Krzysztof