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

[Igor Mammedow <imammedo@redhat.com>]

On Wed, 20 May 2020 07:23:21 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Wed, May 20, 2020 at 01:05:47PM +0200, Igor Mammedow wrote:
> > On Wed, 20 May 2020 06:28:37 -0400
> > "Michael S. Tsirkin" <mst@redhat.com> 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" <mst@redhat.com> 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 <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.    
> > > > 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    
> > > 
> > > It would make sense but is it practical to add the capability is
> > > added in a generic way to all bridges and hosts?
> > > If not how do users probe for presence of the capability?  
> > it probably won't work with native SHPC hotplug (which looks to be
> > incomplete in QEMU anyway), but it should work with ACPI and per
> > port PCIE hotplugs.
> > In case of SHPC, we probably should be able to cleanly error out
> > with 'unsupported' reason if  "hotpluggable" conflicts with bridge
> > policy.  
> 
> "Try it and see if it works" is somewhat problematic from management
> POV since there's a never ending stream of new things they would have
> to try. If this approach is taken, we'd have to try to loop in some
> people from libvirt and see what's their take.
to clarify, we are talking here about bridges to conventional
PCI with native SHPC hotplug semantics wrt mgmt and
potential pcidevice.hotpluggable property.
(the later should work fine in ACPI and PCIE hoptlug cases).

currently by default pci bridges have property shpc=off, so mgmt
should know that deals with PCI bridge and has to enable SHPC
on bridge explicitly, in which case it could probably be taught that
using conflicting hotpluggable for device attached to bridge and shpc
values is wrong thing.
If that's not it, then I'm not sure what kind of discovery you are
talking about.