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

[Daniel P . Berrangé]

On Wed, May 20, 2020 at 07:29:25AM -0400, Michael S. Tsirkin wrote:
> On Wed, May 20, 2020 at 11:06:28AM +0100, Daniel P. Berrangé wrote:
> > On Wed, May 20, 2020 at 11:56:26AM +0200, Igor Mammedow wrote:
> > > On Wed, 20 May 2020 05:47:53 -0400
> > > "Michael S. Tsirkin" <address@hidden> wrote:
> > > 
> > > > On Wed, May 20, 2020 at 11:43:54AM +0200, Igor Mammedow wrote:
> > > > > On Fri, 15 May 2020 12:13:53 +0000
> > > > > Ani Sinha <address@hidden> wrote:
> > > > >   
> > > > > > > On May 14, 2020, at 1:13 AM, Igor Mammedov <address@hidden>
> > > > > > > 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.
> > > Question was more about external interface, were we do not have ports
> > > as separate devices with conventional PCI. The only knob we have is a
> > > a PCI device, where we have a property to turn on/off hotplug. ex:
> > >   -device e1000,hotpluggable=off
> > > and if libvirt would be able to use it
> > 
> > Libvirt can certainly use a property that is settable per-device,
> > instead of against the pcie-root-port.  In fact the application
> > that requested hotplug control (KubeVirt) would strongly prefer
> > if this was doable per-device, because they really dislike the
> > idea of having to deal with pcie-root-port devices.
> > 
> > So if you added it per-device, libvirt would support both the
> > per-device option, and the pcie-root-port option.
> 
> The issue is that there then is no a way to check whether the option
> is supported on a given slot, without simply trying.
> This is why we added it on the port, there, presence of the option
> is a signal that it will work.
> Is this a concern for libvirt?

I'm not sure. What are the scenarios in which it is supported vs not
supported, as I'm unclear if they affect libvirt or not ?


Regards,
Daniel
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