Re: [Qemu-devel] [PATCH v2 1/1] KVM s390 pci infrastructure modelling

[Hong Bo Li <lihbbj@linux.vnet.ibm.com>]

On 7/6/2015 18:56, Michael S. Tsirkin wrote:
> On Mon, Jul 06, 2015 at 10:06:50AM +0800, Hong Bo Li wrote:
>>
>> On 7/5/2015 2:25, Michael S. Tsirkin wrote:
>>> On Fri, Jul 03, 2015 at 07:09:59PM +0800, Hong Bo Li wrote:
>>>>> But I would like to note that pci device drivers require driver handshake
>>>>> before device goes away.
>>>>> IIUC s390 hotplug is immediate, which is a problem.
>>>>> Maybe doing the change will help make sure device removal is acked
>>>>> by guest before it happens?
>>>>>
>>>> I did some prototype today. If define zpci first, the progress of unplug
>>>> will get complicated.
>>> The point is that you don't have to remove the zpci device at all.
>>> Remove pci device from zpci.
>>>
>>> I think the complication you refer to is the guest ack of
>>> the removal, isn't it?
>>> It's complicated, but it has a chance to actually work with
>>> pci device drivers.
>>>
>>> This, as opposed to just removing the device whenever host
>>> tells us to.
>> This patch supports the ack in this way:
>> After unplugging, the guest will do some cleanup work and disable the zpci device.
>> The "is_unplugged" flag in this patch is used to do this ack. Only after the device
>> be disabled, we can remove the zpci device from list and do unparent.
>>
>> The complication I mean is:
>> 1. If we define zpci first, the user can unplug a s390 pci device in two ways:
>>      a) unplug the vfio pci device first, unplug the zpci device second.
>>          If the user only tell us to unplug the vfio pci, after the ack, we will
>> 	still need to wait for the unplug zpci cmd from user,  before that,
>> 	we have to maintain a useless zpci in list.
>>
>>      b) Unplug the zpci device directly. This will cause the unplugging of vfio pci
>> 	automatically. Then on s390, we have a different unplug cmd comparing to
>> 	other platform.
>>
>> 2. If we define vfio pci first,  the user can unplug a s390 pci device in two ways:
>>      a) Unplug the zpci first, unplug the vfio pci device second.
>>         We don't need to maintain the extra s390 zpci structure, after ack, we can
>> 	remove the zpci from list and do unparent.
>>      b) Unplug the vfio pci directly. This will cause the unplugging of zpci
>>         automatically.  Then on s390, we have a same unplug cmd comparing to
>>         other platform.
> You can do the automatic unplug of zpci in 1 as well, can you not?

ok, got your point, will make a patch to verify that. Thank you!

>
>> The ack of these two methods are the same.
>>
>>
>>>> So I prefer defining vfio pci first.
>>>> And it looks like the vfio pci is the basic device, if we want this
>>>> vfio pci to work on s390, we have to define a zpci device to give some
>>>> additional information to it.
>>> if vfio connects to the bus internal to zpci, it can get
>>> things from the bus in a natural way.
>>>
>>> If zpci is connected to vfio, it becomes much messier.
>>>
>> For these two ways, the vfio pci both connect to the s390 pci root bus.
>> And zpci devices connect to the s390-pci-fac-bus, there is no difference.
> What if you don't specify zpci? Does vfio still work?
> If no then we have to conclude that vfio is connected through
> zpci not alongside it.
>