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

[Michael S. Tsirkin]

On Wed, Apr 29, 2020 at 07:43:04AM +0000, Ani Sinha wrote:
> 
> 
> > 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

That doesn't seem to require that hotplug keeps working.

> 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
>