linux-kernel - Re: [PATCH] pciehp: fast unplug for virtual machines

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Date:   Fri, 12 Nov 2021 01:00:43 +0100
From:   Krzysztof Wilczyński <kw@...ux.com>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     linux-pci@...r.kernel.org, mst@...hat.com,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pciehp: fast unplug for virtual machines

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