Re: [virtio-dev] Re: [PATCH v9 02/10] admin: introduce device group and related concepts

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 24, 2022 at 03:37:42PM +0800, Jason Wang wrote:
> On Thu, Nov 24, 2022 at 3:08 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Thu, Nov 24, 2022 at 01:41:41PM +0800, Jason Wang wrote:
> > > On Thu, Nov 24, 2022 at 5:08 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > >
> > > > Each device group has a type. For now, define one initial group:
> > > >
> > > > SR-IOV type - PCI SR-IOV virtual functions (VFs) of a given
> > > > PCI SR-IOV physical function (PF). This group may contain one or more
> > > > virtio devices.
> > > >
> > > > Each device within a group has a unique identifier. This identifier
> > > > is the group member identifier.
> > > >
> > > > Note: one can argue both ways whether the new device group handling
> > > > functionality (this and following patches) is closer
> > > > to a new device type or a new transport type.
> > > >
> > > > However, I expect that we will add more features in the near future. To
> > > > facilitate this as much as possible of the text is located in the new
> > > > admin chapter.
> > > >
> > > > I did my best to minimize transport-specific text.
> > > >
> > > > Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
> > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > > ---
> > > >  admin.tex   | 45 +++++++++++++++++++++++++++++++++++++++++++++
> > > >  content.tex |  2 ++
> > > >  2 files changed, 47 insertions(+)
> > > >  create mode 100644 admin.tex
> > > >
> > > > diff --git a/admin.tex b/admin.tex
> > > > new file mode 100644
> > > > index 0000000..6ebdd05
> > > > --- /dev/null
> > > > +++ b/admin.tex
> > > > @@ -0,0 +1,45 @@
> > > > +\section{Device groups}\label{sec:Basic Facilities of a Virtio Device / Device groups}
> > > > +
> > > > +It is occasionally useful to have a device control a group of
> > > > +other devices. Terminology used in such cases:
> > > > +
> > > > +\begin{description}
> > > > +\item[Device group]
> > > > +        or just group, includes zero or more devices.
> > > > +\item[Owner device]
> > > > +        or owner, the device controlling the group.
> > > > +\item[Member device]
> > > > +        a device within a group. The owner device itself is not
> > > > +       a member of the group.
> > > > +\item[Member identifier]
> > > > +        each member has this identifier, unique within the group
> > > > +       and used to address it through the owner device.
> > > > +\item[Group type identifier]
> > > > +       specifies what kind of member devices there are in a
> > > > +       group, how is the member identifier is interpreted
> > > > +       and what kind of control the owner has.
> > > > +       A given owner can control a single group of a given type,
> > > > +       thus the type and the owner together identify the group.
> > > > +\end{description}
> > > > +
> > > > +The following group types, and their identifiers, are currently specified):
> > > > +\begin{description}
> > > > +\item[SR-IOV group type (0x1)]
> > > > +This device group has a PCI Single Root I/O Virtualization
> > > > +(SR-IOV) physical function (PF) device as the owner and includes
> > > > +all its SR-IOV virtual functions (VFs) as members (see
> > > > +\hyperref[intro:PCIe]{[PCIe]}).
> > >
> > > So I wonder what's the advantage of using a global identifier over the
> > > transport specific one. There's almost no way for CCW/MMIO to use
> > > SR-IOV. Limiting it to PCI seems much easier and avoids layer
> > > violation.
> > >
> > > Thanks
> >
> > So we burn up an identifier, ccw and mmio won't be able to use it.
> > Big deal? Why?
> 
> Because it is transport specific. The basic facility should be
> transport independent.

I tried this but the result is spread all over the spec
and does not result in a readable cohesive whole.
So we give up on the transport independent purity a bit and it
seems worth it.
Also explained this in the cover letter - have you seen that?



> > And I think we might find a use for this with MMIO
> > down the road with some kind of passthrough. Who knows.
> 
> Probably, but can it be modeled exactly as what SR-IOV looks like? Or
> anyhow, it's not too late to define this for MMIO at that time. For
> example, we know MSI-X may work for MMIO but we define it only for PCI
> now.
> 
> Thanks

Right. So if we reserve the id for all transports then it will
be easy to do.


> >
> >
> >
> > > > +
> > > > +The PF device itself is not a member of the group.
> > > > +
> > > > +The group type identifier for this group is 0x1.
> > > > +
> > > > +A member identifier for this group can have a value 0x1 to 0xFFFF
> > > > +and equals the SR-IOV VF number of the member device (see
> > > > +\hyperref[intro:PCIe]{[PCIe]}).
> > > > +
> > > > +Both owner and member devices for this group type use the Virtio
> > > > +PCI transport (see \ref{sec:Virtio Transport Options / Virtio Over PCI Bus}).
> > > > +\end{description}
> > > > +
> > > > +
> > > > diff --git a/content.tex b/content.tex
> > > > index e3203be..3f3585d 100644
> > > > --- a/content.tex
> > > > +++ b/content.tex
> > > > @@ -491,6 +491,8 @@ \section{Exporting Objects}\label{sec:Basic Facilities of a Virtio Device / Expo
> > > >  types. It is RECOMMENDED that devices generate version 4
> > > >  UUIDs as specified by \hyperref[intro:rfc4122]{[RFC4122]}.
> > > >
> > > > +\input{admin.tex}
> > > > +
> > > >  \chapter{General Initialization And Device Operation}\label{sec:General Initialization And Device Operation}
> > > >
> > > >  We start with an overview of device initialization, then expand on the
> > > > --
> > > > MST
> > > >
> >
> >
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