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

[Alexander Graf <agraf@suse.de>]

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