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Re: [Qemu-ppc] [PATCH v1 2/7] pcihp: overwrite hotplug handler recursive

From: Igor Mammedov
Subject: Re: [Qemu-ppc] [PATCH v1 2/7] pcihp: overwrite hotplug handler recursively from the start
Date: Fri, 2 Nov 2018 14:00:32 +0100

On Fri, 2 Nov 2018 12:43:10 +0100
David Hildenbrand <address@hidden> wrote:

> On 01.11.18 15:10, Igor Mammedov wrote:
> > On Wed, 24 Oct 2018 12:19:25 +0200
> > David Hildenbrand <address@hidden> wrote:
> >   
> >> For now, the hotplug handler is not called for devices that are
> >> being cold plugged. The hotplug handler is setup when the machine
> >> initialization is fully done. Only bridges that were cold plugged are
> >> considered.
> >>
> >> Set the hotplug handler for the root piix bus directly when realizing.
> >> Overwrite the hotplug handler of bridges when hotplugging/coldplugging
> >> them.
> >>
> >> This will now make sure that the ACPI PCI hotplug handler is also called
> >> for cold-plugged devices (also on bridges) and for bridges that were
> >> hotplugged.
> >>
> >> When trying to hotplug a device to a hotplugged bridge, we now correctly
> >> get the error message
> >>  "Unsupported bus. Bus doesn't have property 'acpi-pcihp-bsel' set"
> >> Insted of going via the standard PCI hotplug handler.  
> > Erroring out is probably not ok, since it can break existing setups
> > where SHPC hotplugging to hotplugged bridge was working just fine before.  
> The question is if it actually was supposed (and eventually did) work.
I think it works now, it's QEMU 'ACPI hotplug hack' (which exists for
the sake of Windows) limitation. We weren't able to dynamically add
ACPI description for hotplugged bridge, so it was using native hotplug.
Now theoretically we can load tables dynamically but that, would add
maintenance nightmare (versioned tables) and would be harder to debug.
I'd rather not go that direction and keep current limited version,
suggesting users to use native hotplug if guest is capable.

> If this was the expected behavior (mixing hotplug types), then the
> necessary change to this patch would boil down to checking if the bridge
> it hot or coldplugged.
> > 
> > Marcel/Michael what's your take on this change in behaviour?
> > CCing libvirt in case they are doing this stuff
> >   
> Indeed, it would be nice to know if this was actually supposed to work
> like this (coldplugged bridges using ACPI hotplug and hotplugged bridges
> using SHPC hotplug).

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