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

[Ani Sinha]

> On Apr 29, 2020, at 1:08 PM, Michael S. Tsirkin <address@hidden> wrote:
> 
> On Wed, Apr 29, 2020 at 07:02:56AM +0000, Ani Sinha wrote:
>> 
>> 
>>> On Apr 29, 2020, at 12:27 PM, Michael S. Tsirkin <address@hidden> wrote:
>>> 
>>> On Wed, Apr 29, 2020 at 06:54:52AM +0000, Ani Sinha wrote:
>>>> 
>>>> 
>>>>> On Apr 29, 2020, at 12:22 PM, Michael S. Tsirkin <address@hidden> wrote:
>>>>> 
>>>>> On Wed, Apr 29, 2020 at 06:11:20AM +0000, Ani Sinha wrote:
>>>>>> 
>>>>>> 
>>>>>>> On Apr 29, 2020, at 10:58 AM, Michael S. Tsirkin <address@hidden> wrote:
>>>>>>> 
>>>>>>> o if there's a need to disable
>>>>>>> just one of these, commit log needs to do a better job documenting the
>>>>>>> usecase.
>>>>>> 
>>>>>> The use case is simple. With this feature admins will be able to do what 
>>>>>> they were forced to do from Windows driver level but now at the bus 
>>>>>> level. Hence, 
>>>>>> (a) They need not have access to the guest VM to change or update 
>>>>>> windows driver registry settings. They can enable the same setting from 
>>>>>> admin management console without any access to VM.
>>>>>> (b) It is more robust. No need to mess with driver settings. Incorrect 
>>>>>> settings can brick guest OS. Also no guest specific knowhow required.
>>>>>> (c) It is more scalable since a single cluster wide setting can be used 
>>>>>> for all VM power ons and the management plane can take care of the rest 
>>>>>> automatically. No need to access individual VMs to enforce this.
>>>>>> (d) I am told that the driver level solution does not persist across a 
>>>>>> reboot. 
>>>>>> 
>>>>>> Ani
>>>>> 
>>>>> Looks like disabling both plug and unplug would also address these needs.
>>>> 
>>>> No the driver level solution does not prevent hot plugging of devices but 
>>>> blocks just hot unplugging. The solution I am proposing tries to do the 
>>>> same but from the bus/hypervisor level.
>>> 
>>> Why does the driver level solution need to prevent just hot unplugging?
>> 
>> Because it not fair to prevent end users from hot plugging new devices when 
>> it is the (accidental?) hot unplugging of existing devices which causes 
>> issues.
> 
> Accidental? So maybe what you need is actually something else then -
> avoid *removing* the device when it's powered down.

You don’t get it. It is not hypervisor admins who are unplugging it. It is the 
end users. Even RedHat customers want this feature. See following resources: 
https://www.redhat.com/archives/libvir-list/2020-February/msg00110.html
https://bugzilla.redhat.com/show_bug.cgi?id=1802592
https://bugzilla.redhat.com/show_bug.cgi?id=1790899

My approach is much more fine grained than just disable everything approach 
that we have for q35. For i440fx we can do better than that.



> 
>>> 
>>> 
>>>> 
>>>>> -- 
>>>>> MST