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

[Ani Sinha]

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


>
> > ---
> >  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
>
>