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

[Igor Mammedov]

On Fri, 12 Nov 2021 12:15:28 +0100
Gerd Hoffmann <kraxel@redhat.com> wrote:

> On Thu, Nov 11, 2021 at 10:39:59AM -0500, Michael S. Tsirkin wrote:
> > On Thu, Nov 11, 2021 at 01:09:05PM +0100, Gerd Hoffmann wrote:  
> > >   Hi,
> > >   
> > > > When the acpihp driver is used the linux kernel will just call the aml
> > > > methods and I suspect the pci device will stay invisible then because
> > > > nobody flips the slot power control bit (with native-hotplug=on, for
> > > > native-hotplug=off this isn't a problem of course).  
> > > 
> > > Hmm, on a quick smoke test with both patch series (mine + igors) applied
> > > everything seems to work fine on a quick glance.  Dunno why.  Maybe the
> > > pcieport driver turns on slot power even in case pciehp is not active.  
> 
> Digged deeper.  Updating power status is handled by the plug() callback,
> which is never called in case acpi hotplug is active.  The guest seems
> to never touch slot power control either, so it's working fine.  Still
> feels a bit fragile though.
> 
> > Well power and hotplug capabilities are mostly unrelated, right?  
> 
> At least they are separate slot capabilities.  The linux pciehp driver
> checks whenever the power control is present before using it, so having
> PwrCtrl- HotPlug+ seems to be a valid combination.
> 
> We even have an option for that: pcie-root-port.power_controller_present
> 
> So flipping that to off in case apci hotplug is active should make sure
> we never run into trouble with pci devices being powered off.
> 
> Igor?  Can you add that to your patch series?

Sorry, saw it too late.
I'll test idea with my set of guests to see if there are any adverse effects.


> > I feel switching to native so late would be inappropriate, looks more
> > like a feature than a bugfix. Given that - we need Igor's patches.
> > Given that - would you say I should apply yours?  
> 
> I think when setting power_controller_present=off for acpi hotplug it is
> safe to merge both mine and igor's.
> 
> take care,
>   Gerd
>