Re: RFC: vfio-pci API for PCI bus/slot (hot) resets

From: Alex Williamson
Date: Fri Aug 02 2013 - 19:37:55 EST

On Sat, 2013-08-03 at 09:15 +1000, Benjamin Herrenschmidt wrote:
> On Fri, 2013-08-02 at 16:49 -0600, Bjorn Helgaas wrote:
> > [+cc linux-pci]
> >
> > On Fri, Aug 2, 2013 at 3:28 PM, Benjamin Herrenschmidt
> > <benh@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > > Right. Another use case is, I know of devices that need a fundamental
> > > reset (PERST) after applying a FW update.
> >
> > This is a tangent from the real discussion here, but the question of
> > resetting a device after a firmware update concerns me. Many if not
> > all of our current reset interfaces save & restore the architected
> > parts of config space around the reset. But a reset after a firmware
> > update may change things like the number and type of BARs or even the
> > functionality of the device, so I don't think the restore is safe in
> > general.
> Right.
> > I doubt this is a big problem in general, but I have found reports of
> > people having to do a system reset or reboot after updating, e.g.,
> > FPGA images. I suppose at least some of these could be worked around
> > with the right hotplug incantations.
> Yes.
> We have that similar issue with error handling, when the driver doesn't
> have the right hooks, we simulate an unplug, reset, then replug.
> Maybe we could provide generic helpers to do that...

Devices going away and coming back is pretty difficult for vfio to
handle. Perhaps helpers to rescan a device in-place would be easier.
On the QEMU side we'd need to rescan the device after each reset, which
would be rather tedious for the typical case where it doesn't change.


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