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

[Marcel Apfelbaum]

Hi Ani,

On 4/18/20 6:25 AM, Ani Sinha wrote:
> +Julia who implemented https://patchwork.kernel.org/patch/11388881/
>
>
> > On Apr 18, 2020, at 3:26 AM, Laine Stump <address@hidden> wrote:
> >
> > On 4/17/20 12:35 PM, Ani Sinha wrote:
> > > +Laine
> > > > On Apr 17, 2020, at 9:39 PM, Michael S. Tsirkin <address@hidden> wrote:
> > > >
> > > > Problem is, I think this is not something we can support with pcie or shpc.
> > > > I'm reluctant to add features that only ACPI can support,
> > > > we are trying to phase that out.
> > > Hmm. I see. We use conventional PCI and hence was looking for providing this 
> > > feature for conventional PCI only. Laine might be able to throw some lights as 
> > > to feasibility of the in PCIE world.
> > Sorry, my knowledge doesn't go that low. If there's a qemu option I can expose 
> > it in libvirt, but am by no means an expert of qemu internals or the pci/pcie 
> > specs :-)
> >
> > (BTW, I think in the past people have prevented enabling hot-unplug by unprivileged users 
> > in Windows with some sort of a "system policy" in Windows. (whatever that is - 
> > I don't use Windows, and have just heard this from others when discussing the problem).

A PCIe Root Port or a PCI slot can or cannot support hot-plugging. 
Anything in the middle can't be done at PCIe/PCI level (as far as I know).
I think the answer can be at the modelling level. Use non hot-pluggable 
slots (or PCIe Root Ports without hot-plug support) for the devices
you don't want hot-unplugged, leave an empty PCI Bridge (or some PCIe 
Root Ports with hot-plug support) to be able to hot-plug devices.

Thanks,
Marcel

> My question to Julia is whether it is possible to disable just hot unplug but 
> keep hot plugging for PCIE devices enabled using PCI_EXP_SLTCAP_HPC  or 
> otherwise in Qemu?