Re: [PATCH 0/6] RfC: try improve native hotplug for pcie root ports

[Michael S. Tsirkin]

On Thu, Nov 11, 2021 at 09:35:37AM +0000, Daniel P. Berrangé wrote:
> On Thu, Nov 11, 2021 at 03:20:07AM -0500, Michael S. Tsirkin wrote:
> > On Thu, Nov 11, 2021 at 08:53:06AM +0100, Gerd Hoffmann wrote:
> > >   Hi,
> > > 
> > > > Given it's a bugfix, and given that I hear through internal channels
> > > > that QE results so far have been encouraging, I am inclined to bite the
> > > > bullet and merge this for -rc1.
> > > 
> > > Fine with me.
> > > 
> > > > I don't think this conflicts with Julia's patches as users can still
> > > > disable ACPI hotplug into bridges.  Gerd, agree?  Worth the risk?
> > > 
> > > Combining this with Julia's patches looks a bit risky to me and surely
> > > needs testing.  I expect the problematic case is both native and acpi
> > > hotplug being enabled.
> > >  When the guest uses acpi hotplug nobody will
> > > turn on slot power on the pcie root port ...
> > 
> > I'm not sure I understand what the situation is, and how to trigger it.
> > Could you clarify pls?
> > 
> > > I'd suggest to just revert to pcie native hotplug for 6.2.
> > 
> > Hmm that kind of change seems even riskier to me. I think I'll try with
> > Igor's patches.
> 
> Why would it be risky ? PCIE native hotplug is what we've used in
> QEMU for years & years, until 6.1 enabled the buggy ACPI hotplug.
> The behaviour of the current PCIE native hotplug impl is a known
> quantity.
> 
> Regards,
> Daniel

For example we might regress some of the bugs that the switch to ACPI fixed 
back to
6.0 state. There were a bunch of these. For example it should be
possible for guests to disable native and use ACPI then, but isn't.
I'm very willing to consider the switch back to native by default
but given the timing doing big changes like that at the last
minute seems unusual.

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