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

[Jason Wang <jasowang@redhat.com>]

å 2022/11/24 20:12, Cornelia Huck åé:
> On Thu, Nov 24 2022, "Michael S. Tsirkin" <mst@redhat.com> wrote:
>
> > 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:
> > > > > > +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.


I may miss something, but it looks to me it's just a subsection in the 
PCI transport to describe the SR-IOV group identifier.


> > 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?


Sorry, I don't.


> > > > 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.
> Also, if we go with transport-specific ids, we might end up with
> different ids per transport when we add a future transport-independent
> feature.


I'm not sure this is a real problem:

1) all the basic facilities are now transport-independent but they are 
implemented via different transport specific registers/commands/offsets etc.
2) I'm not quite sure there would be a transport-independent group 
identifier other than the virtqueue transport, so I wonder if it makes 
sense to split the id space into global ones as well as the transport 
specific ones


>   The global id space really should be big enough to accommodate
> even single-transport groups.


Yes, so it's not about the space but about whether or it it's 
worth/correct/expensive to describe a transport specific identifier in 
the basic facility part.

Thanks


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