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

Michael S. Tsirkin
2021-11-14T17:25:09Z

On Sun, Nov 14, 2021 at 05:39:58PM +0100, Lukas Wunner wrote:
> On Thu, Nov 11, 2021 at 10:02:24AM +0100, Gerd Hoffmann wrote:
> > The PCIe specification asks the OS to wait five seconds after the
>
> The spec reference Bjorn asked for is: PCIe r5.0, sec. 6.7.1.5
>
> > 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.
>
> Why does virtual hardware implement the Attention Button if it's
> perceived as annoying? Just amend qemu so that it doesn't advertise
> presence of an Attention Button to get rid of the delay. (Clear the
> Attention Button Present bit in the Slot Capabilities register.)

Because we want ability to request device removal from outside the
guest.

> An Attention Button doesn't make any sense for virtual hardware
> except to test or debug support for it in the kernel. Just make
> presence of the Attention Button optional and be done with it.
>
> You'll still be able to bring down the slot in software via the
> "remove" attribute in sysfs.

This requires guest specific code though. Emulating the attention button
works in a guest independent way.

> Same for the 1 second delay in remove_board(). That's mandated by
> PCIe r5.0, sec. 6.7.1.8, but it's only observed if a Power Controller
> is present. So just clear the Power Controller Present bit in the
> Slot Capabilities register and the delay is gone.
>
>
> > @@ -109,6 +110,8 @@ struct controller {
> > unsigned int ist_running;
> > int request_result;
> > wait_queue_head_t requester;
> > +
> > + bool is_virtual;
> > };
>
> This is a quirk for a specific device, so please move it further up to the
> /* capabilities and quirks */ section of struct controller.
>
>
> > @@ -227,6 +227,11 @@ static int pciehp_probe(struct pcie_device *dev)
> > goto err_out_shutdown_notification;
> > }
> >
> > + if (dev->port->vendor == PCI_VENDOR_ID_REDHAT &&
> > + dev->port->device == 0x000c)
> > + /* qemu pcie root port */
> > + ctrl->is_virtual = true;
> > +
>
> Move this to pcie_init() in pciehp_hpc.c below the existing quirks for
> hotplug_user_indicators and is_thunderbolt.
>
>
> > +static bool fast_virtual_unplug = true;
> > +module_param(fast_virtual_unplug, bool, 0644);
>
> An integer parameter to configure a custom delay would be nicer IMO.
> Of course, anything else than 5 sec deviates from the spec.
>
> Thanks,
>
> Lukas