Re: [PATCH v2 3/3] vfio/pci: Don't probe devices that can't be reset

[David Daney]

On 08/18/2017 07:12 AM, Alex Williamson wrote:
> On Fri, 18 Aug 2017 15:42:31 +0200
> Jan Glauber <jan.glauber@xxxxxxxxxxxxxxxxxx> wrote:
>
> > On Thu, Aug 17, 2017 at 07:00:17AM -0600, Alex Williamson wrote:
> > > On Thu, 17 Aug 2017 10:14:23 +0200
> > > Jan Glauber <jglauber@xxxxxxxxxx> wrote:
> > >    
> > > > If a PCI device supports neither function-level reset, nor slot
> > > > or bus reset then refuse to probe it. A line is printed to inform
> > > > the user.
> > >
> > > But that's not what this does, this requires that the device is on a
> > > reset-able bus.  This is a massive regression.  With this we could no
> > > longer assign devices on the root complex or any device which doesn't
> > > return from bus reset and currently makes use of the NO_BUS_RESET flag
> > > and works happily otherwise.  Full NAK.  Thanks,
> >
> > Looks like I missed the slot reset check. So how about this:
> >
> > if (pci_probe_reset_slot(pdev->slot) && pci_probe_reset_bus(pdev->bus)) {
> > 	dev_warn(...);
> > 	return -ENODEV;
> > }
> >
> > Or am I still missing something here?
>
> We don't require that a device is on a reset-able bus/slot, so any
> attempt to impose that requirement means that there are devices that
> might work perfectly fine that are now excluded from assignment.  The
> entire premise is unacceptable.  Thanks,


You previously rejected the idea to silently ignore bus reset requests 
on buses that do not support it.

So this leaves us with two options:

1) Do nothing, and crash the kernel on systems with bad combinations of 
PCIe target devices and cn88xx when vfio_pci is used.

2) Do something else.

We are trying to figure out what that something else should be.  The 
general concept we are working on is that if vfio_pci wants to reset a 
device, *and* bus reset is the only option available, *and* cn88xx, then 
make vfio_pci fail.

What is your opinion of doing that (assuming it is properly implemented)?

Thanks,
David Daney