Re: [Qemu-devel] [PATCH RFC 1/1] s390x/pci: Extend pci representation by new zpci device

[Frank Blaschka <blaschka@linux.vnet.ibm.com>]

On Wed, Mar 04, 2015 at 03:49:15PM +0100, Alexander Graf wrote:
> 
> 
> On 04.03.15 14:44, Frank Blaschka wrote:
> > On Tue, Mar 03, 2015 at 09:38:37PM +0100, Alexander Graf wrote:
> >>
> >>
> >> On 03.03.15 14:25, Frank Blaschka wrote:
> >>> On Tue, Mar 03, 2015 at 10:33:05AM +0100, Alexander Graf wrote:
> >>>>
> >>>>
> >>>>
> >>>>> Am 03.03.2015 um 09:06 schrieb Frank Blaschka <blaschka@linux.vnet.ibm.com>:
> >>>>>
> >>>>>> On Thu, Feb 26, 2015 at 04:34:06PM +0100, Alexander Graf wrote:
> >>>>>>
> >>>>>>
> >>>>>>> On 26.02.15 16:27, Frank Blaschka wrote:
> >>>>>>>> On Thu, Feb 26, 2015 at 03:39:15PM +0100, Alexander Graf wrote:
> >>>>>>>>
> >>>>>>>>
> >>>>>>>>> On 26.02.15 12:59, Frank Blaschka wrote:
> >>>>>>>>> This patch extends the current s390 pci implementation to
> >>>>>>>>> provide more flexibility in configuration of s390 specific
> >>>>>>>>> device handling. For this we had to introduce a new facility
> >>>>>>>>> (and bus) to hold devices representing information actually
> >>>>>>>>> provided by s390 firmware and I/O configuration.
> >>>>>>>>>
> >>>>>>>>> On s390 the physical structure of the pci system (bridge, bus, slot)
> >>>>>>>>> in not shown to the OS. For this the pci bridge and bus created
> >>>>>>>>> in qemu can also not be shown to the guest. The new zpci device class
> >>>>>>>>> represents this abstract view on the bare pci function and allows to
> >>>>>>>>> provide s390 specific configuration attributes for it.
> >>>>>>>>>
> >>>>>>>>> Sample qemu configuration:
> >>>>>>>>> -device e1000,id=zpci1
> >>>>>>>>> -device ne2k_pci,id=zpci2
> >>>>>>>>> -device zpci,fid=2,uid=1248,pci_id=zpci1
> >>>>>>>>> -device zpci,fid=17,uid=2244,pci_id=zpci2
> >>>>>>>>>
> >>>>>>>>> A zpci device references the corresponding PCI device via device id.
> >>>>>>>>> The new design allows to define multiple host bridges and support more
> >>>>>>>>> pci devices.
> >>>>>>>>
> >>>>>>>> Isn't this reverse? Shouldn't it rather be
> >>>>>>>>
> >>>>>>>>  -device zpci,...,id=zpci1
> >>>>>>>>  -device e1000,bus=zpci1.0
> >>>>>>>>
> >>>>>>>> with a limit on each virtual zpci bus to only support one device?
> >>>>>>>
> >>>>>>> Do you mean something like having multiple host bridges (providing a pci bus
> >>>>>>> each) and limit the bus to just one device?
> >>>>>>>
> >>>>>>> -device s390-pcihost,fid=16,uid=1234
> >>>>>>> -device s390-pcihost,fid=17,uid=5678
> >>>>>>> -device e1000,bus=pci.0
> >>>>>>> -device ne2k_pci,bus=pci.1
> >>>>>>>
> >>>>>>> We also discussed this option but we don't like the idea to put attributes
> >>>>>>> belong to the pci device to the host bridge.
> >>>>>>
> >>>>>> I guess I'm not grasping something obvious here :). What exactly are the
> >>>>>> attributes again?
> >>>>> Sorry for the late response, I was on vacation the last couple days.
> >>>>>
> >>>>> The fid and uid values are provided by microcode/io layer on the real hardware.
> >>>>
> >>>> So they are arbitrary numbers? What uniqueness constraints do we have on them?
> >>> fid and uid must be unique within the same qemu. At a first look the numbers are
> >>> arbitrary but our configuration folks want explicitly define a particular fid and uid
> >>> to better support migration and pass-through scenarios.
> >>
> >> Well, at the end of the day you want to make sure they're identical on
> >> both sides, yes.
> >>
> >>>> IIUC you can only have a single pcie device behind a virtual "bus" anyway, so what if we just calculate uid and fid from the bus id?
> >>> I think this similar to the current implementation. We use the slot (idea for the future was
> >>> bus + slot) to generate uid and fid. But this is not flexible enough. As I said, our
> >>> configuration folks want to be able to specify fid and uid for the device.
> >>
> >> I don't see how this is different from what PPC does with its LIOBN
> >> which is a property of the PHB.
> >>
> >>
> >> Alex
> >>
> > 
> > I played arround with the idea of having multiple host bridges and this worked well
> > at least for static (non hotplug) configuration. In case I want to hotplug a host
> > bridge I got following error:
> > 
> > (qemu) device_add s390-pcihost,fid=8,uid=9
> > Bus 'main-system-bus' does not support hotplugging
> > 
> > Is there anything I have to enable to support this?
> > 
> > I have: has_dynamic_sysbus = 1 and cannot_instantiate_with_device_add_yet = false
> > but this seems not to help for the hotplug case.
> 
> Having s390 devices reside on sysbus is probably a bad idea. Instead,
> they should be on an s390 specific bus which then can implement hotplug
> easily.
> 
> 
> Alex
>

Hm now I get lost ...

Do you suggest we should implement a s390 specific device (which is not derived from
TYPE_PCI_HOST_BRIDGE) but implements a pci bus so we can attach a pci device to this
device?