Re: [PATCH V2] Add a new PIIX option to control PCI hot unplugging of devices on non-root buses

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, May 20, 2020 at 11:43:54AM +0200, Igor Mammedow wrote:
> On Fri, 15 May 2020 12:13:53 +0000
> Ani Sinha <ani.sinha@nutanix.com> wrote:
> 
> > > On May 14, 2020, at 1:13 AM, Igor Mammedov <imammedo@redhat.com>
> > > wrote: 
> > >> 
> > >>   
> > >>> Will following hack work for you?
> > >>> possible permutations
> > >>> 1) ACPI hotplug everywhere
> > >>> -global PIIX4_PM.acpi-pci-hotplug=on -global
> > >>> PIIX4_PM.acpi-pci-hotplug-with-bridge-support=on -device
> > >>> pci-bridge,chassis_nr=1,shpc=doesnt_matter -device
> > >>> e1000,bus=pci.1,addr=01,id=netdev1 
> > >>> 
> > >>> 2) No hotplug at all
> > >>> -global PIIX4_PM.acpi-pci-hotplug=off -global
> > >>> PIIX4_PM.acpi-pci-hotplug-with-bridge-support=on -device
> > >>> pci-bridge,chassis_nr=1,shpc=off -device
> > >>> e1000,bus=pci.1,addr=01,id=netdev1
> > >>> 
> > >>> -global PIIX4_PM.acpi-pci-hotplug=off -global
> > >>> PIIX4_PM.acpi-pci-hotplug-with-bridge-support=off -device
> > >>> pci-bridge,chassis_nr=1,shpc=doesnt_matter  -device
> > >>> e1000,bus=pci.1,addr=01,id=netdev1    
> > >> 
> > >> Given that my patch is not acceptable, I’d prefer the following in
> > >> the order of preference:
> > >> 
> > >> (a) Have an option to disable hot ejection of PCI-PCI bridge so
> > >> that Windows does not even show this HW in the “safely remove HW”
> > >> option. If we can do this then from OS perspective the GUI options
> > >> will be same as what is available with PCIE/q35 - none of the
> > >> devices will be hot ejectable if the hot plug option is turned off
> > >> from the PCIE slots where devices are plugged into. I looked at
> > >> the code. It seems to manipulate ACPI tables of the empty slots of
> > >> the root bus where no devices are attached (see comment "/* add
> > >> hotplug slots for non present devices */ “). For cold plugged
> > >> bridges, it recurses down to scan the slots of the bridge. Is it
> > >> possible to disable hot plug for the slot to which the bridge is
> > >> attached?  
> > > 
> > > I don't think it's possible to have per slot hotplug on
> > > conventional PCI hardware. it's per bridge property.  
> > 
> > We add the AMLs per empty slot though. When the pic bridge is
> > attached, we do nothing, just recurse into the bridge slots. That is
> > what I was asking, if it was possible to just disable the AMLs or use
> > some tricks to say that this particular slot is not hotpluggable. I
> > am not sure why Windows is trying to eject the PCI bridge and
> > failing. Maybe something related to this comment?
> > 
> > 
> > /* When hotplug for bridges is enabled, bridges are
> >                               
> >          * described in ACPI separately (see build_pci_bus_end).
> >                                       
> >          * In this case they aren't themselves hot-pluggable.
> >                                       
> >          * Hotplugged bridges *are* hot-pluggable.
> > */
> 
> thinking some more on this topic, it seems that with ACPI hotplug we
> already have implicit non-hotpluggble slot (slot with bridge) while
> the rest are staying hotpluggable.
> 
> So my question is: if it's acceptable to add 'PCIDevice::hotpluggable"
> property to all PCI devices so that user / libvirt could set it to
> false in case they do not want coldplugged device be considered as
> hotpluggable? (this way other devices could be treated the same way as
> bridges)
> 
> [...]


I think Julia already posted a patch adding this to downstream pcie
bridges. Adding this to pci slots sounds like a reasonable thing.

-- 
MST