Re: [PATCH-for-6.2] docs: add a word of caution on x-native-hotplug property for pcie-root-ports

[Igor Mammedov <imammedo@redhat.com>]

It's hardly 6.2 material

On Fri, 26 Nov 2021 11:12:55 +0530 (IST)
Ani Sinha <ani@anisinha.ca> wrote:

> On Thu, 25 Nov 2021, Michael S. Tsirkin wrote:
> 
> > On Thu, Nov 25, 2021 at 05:36:29PM +0530, Ani Sinha wrote:  
> > > x-native-hotplug property, when used in order to disable HPC bit on the PCIE
> > > root ports, can lead to unexpected results from the guest operating system.
> > > Users are strongly advised not to touch this property in order to manipulte the
> > > HPC bit. Add a word of caution in the pcie.txt doc file to document this.
> > >
> > > Signed-off-by: Ani Sinha <ani@anisinha.ca>  
> >
> > Do we want to generally document this for all "x-" options?  
> 
> Yes igor suggested it but I sent this one for two reasons:
> (a) I could not find a place to document this for properties without
> adding a new file. This sounded too bigger a hammer at the present. If you
> can suggest an existing place for documenting this for the property names,
> I will go and add this info there as well.
> 
> (b) I think we need to document this experimental property here regardless
> because this doc deals with hotplug and pcie ports and we had too much of
> a mess with this acpi/pci native switch.
> 
> When things stabilize a bit, Igor suggested elsewhere that we start a
> separate doc just for hotplug and various options we have and at
> that point we can move this info in this new doc.
> 
> https://www.mail-archive.com/libvir-list@redhat.com/msg221746.html

I'd rather put a blanket statement somewhere, like:
 
"x-" prefixed properties are experimental, unstable, internal and
are subject to change/go away without prior notice.
Such properties are not meant for use by users unless explicitly
documented otherwise.

> >  
> > > ---
> > >  docs/pcie.txt | 17 ++++++++++++++++-
> > >  1 file changed, 16 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/docs/pcie.txt b/docs/pcie.txt
> > > index 89e3502075..e1f99f725f 100644
> > > --- a/docs/pcie.txt
> > > +++ b/docs/pcie.txt
> > > @@ -262,11 +262,26 @@ PCI Express Root Ports (and PCI Express Downstream Ports).
> > >          Port, which may come handy for hot-plugging another device.
> > >
> > >
> > > -5.3 Hot-plug example:
> > > +5.2 Hot-plug example:
> > >  Using HMP: (add -monitor stdio to QEMU command line)
> > >    device_add <dev>,id=<id>,bus=<PCI Express Root Port Id/PCI Express Downstream Port Id/PCI-PCI Bridge Id/>
> > >
> > >
> > > +5.3 A word of caution using hotplug on PCI Express Root Ports:
> > > +Starting Qemu version 6.2, PCI Express Root ports have a property
> > > +"x-native-hotplug" ("native-hotplug" for Qemu version 6.1), that can be used to
> > > +enable or disable hotplug on that port. For example:
> > > +
> > > +-device pcie-root-port,x-native-hotplug=off,... etc.
> > > +
> > > +The "x-" prefix indicates that this property is highly experimental and can
> > > +lead to unexpected results from the guest operating system if users try to use
> > > +it to alter the native hotplug on the port. It also means that the property
> > > +name and its behavior is liable to change in the future and is not expected to
> > > +be stable across Qemu versions. Therefore, end users are advised not to change
> > > +the value of this option from its default set value or use it in the Qemu
> > > +command line.
> > > +
> > >  6. Device assignment
> > >  ====================
> > >  Host devices are mostly PCI Express and should be plugged only into
> > > --
> > > 2.25.1  
> >
> >  
>