Re: [PATCH v2] docs/s390x/pcidevices: document pci devices on s390x

[Sebastian Mitterle]

On Fri, Jan 27, 2023 at 11:30 AM Cornelia Huck <cohuck@redhat.com> wrote:
>
> On Fri, Jan 27 2023, Sebastian Mitterle <smitterl@redhat.com> wrote:
>
> > Add some documentation about the zpci device and how
> > to use it with pci devices on s390x.
> >
> > Used source: Cornelia Huck's blog post
> > https://people.redhat.com/~cohuck/2018/02/19/notes-on-pci-on-s390x.html
> >
> > Signed-off-by: Sebastian Mitterle <smitterl@redhat.com>
> > ---
> > v2: move section below 'Device support'
> > ---
> >  docs/system/s390x/pcidevices.rst | 34 ++++++++++++++++++++++++++++++++
> >  docs/system/target-s390x.rst     |  1 +
> >  2 files changed, 35 insertions(+)
> >  create mode 100644 docs/system/s390x/pcidevices.rst
> >
> > diff --git a/docs/system/s390x/pcidevices.rst 
> > b/docs/system/s390x/pcidevices.rst
> > new file mode 100644
> > index 0000000000..fec905d6e6
> > --- /dev/null
> > +++ b/docs/system/s390x/pcidevices.rst
> > @@ -0,0 +1,34 @@
> > +PCI devices on s390x
> > +====================
> > +
> > +PCI devices on s390x work differently than on other architectures.
>
> add "and need to be configured in a slightly different way." ?
>
> > +
> > +To start with, using a PCI device requires the additional ``zpci`` device. 
> > For example,
>
> I think the "zpci" device is not technically "required" (ISTR that we
> autogenerate it, if needed); however, you need it if you actually want
> to specify uid/fid/... what about:
>
> "Every PCI device is linked with an additional ``zpci`` device. While
> the ``zpci`` device will be autogenerated if not specified, it is
> recommended to specify it explicitly so that you can pass s390-specific
> PCI configuration."
>
> ?
>
> > +in order to pass a PCI device ``0000:00:00.0`` through you'd specify::
> > +
> > + qemu-system-s390x ... \
> > +                   -device zpci,uid=1,fid=0,target=hostdev0,id=zpci1 \
> > +                   -device vfio-pci,host=0000:00:00.0,id=hostdev0
> > +
> > +Here, the zpci device is joined with the PCI device via the ``target`` 
> > property.
> > +
> > +Note that we don't set bus, slot or function here for the guest as is 
> > common in other
> > +PCI implementations. Topology information is not available on s390x. 
> > Instead, ``uid``
>
> "Topology information is not available on s390x, and the guest will not
> see any of the bus/slot/function information specified on the command
> line."
>
> ?
>
> > +and ``fid`` determine how the device is presented to the guest operating 
> > system.
> > +
> > +In case of Linux, ``uid`` will be used in the ``domain`` part of the PCI 
> > identifier, and
> > +``fid`` identifies the physical slot, i.e.::
> > +
> > + qemu-system-s390x ... \
> > +                   -device zpci,uid=7,fid=8,target=hostdev0,id=zpci1 \
> > +                   ...
> > +
> > +will be presented in the guest as::
> > +
> > + # lspci -v
> > + 0007:00:00.0 ...
> > + Physical Slot: 00000008
> > + ...
> > +
> > +Finally, note that you might have to enable the ``zpci`` feature in the 
> > cpu model in oder to use
> > +it.
>
> I'm wondering what the current state of that feature is -- is it present
> by default in the newer named models? (My original blog entry was
> written nearly five years ago ;)
>
David Hildenbrand confirmed the feature is default-on in the qemu
model since qemu 4.0 and also for the host model if supported by the
modern hardware. Therefore, I'll omit this sentence.