Re: [virtio-dev] [RFC] content: support SR-IOV

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 18 May 2018 16:43:09 +0800
Tiwei Bie <tiwei.bie@intel.com> wrote:

> On Fri, May 18, 2018 at 10:14:38AM +0200, Cornelia Huck wrote:
> > On Fri, 18 May 2018 15:38:55 +0800
> > Tiwei Bie <tiwei.bie@intel.com> wrote:
> >   
> > > Reserve a feature bit for virtio devices which support SR-IOV.  
> > 
> > Reserving a feature bit for this makes sense, but...
> >   
> > > 
> > > Suggested-by: Michael S. Tsirkin <mst@redhat.com>
> > > Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
> > > ---
> > > More details can be found from this thread:
> > > https://patchwork.kernel.org/patch/10285541/
> > > 
> > >  content.tex | 14 ++++++++++++--
> > >  1 file changed, 12 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/content.tex b/content.tex
> > > index 7a92cb1..62214fa 100644
> > > --- a/content.tex
> > > +++ b/content.tex
> > > @@ -95,10 +95,10 @@ Feature bits are allocated as follows:
> > >  \begin{description}
> > >  \item[0 to 23] Feature bits for the specific device type
> > >  
> > > -\item[24 to 33] Feature bits reserved for extensions to the queue and
> > > +\item[24 to 36] Feature bits reserved for extensions to the queue and
> > >    feature negotiation mechanisms
> > >  
> > > -\item[34 and above] Feature bits reserved for future extensions.
> > > +\item[37 and above] Feature bits reserved for future extensions.
> > >  \end{description}
> > >  
> > >  \begin{note}
> > > @@ -5348,6 +5348,8 @@ Descriptors} and \ref{sec:Packed Virtqueues / Indirect Flag: Scatter-Gather Supp
> > >    \item[VIRTIO_F_IN_ORDER(35)] This feature indicates
> > >    that all buffers are used by the device in the same
> > >    order in which they have been made available.
> > > +  \item[VIRTIO_F_SR_IOV(36)] This feature indicates that
> > > +  the device supports Single Root I/O Virtualization.  
> > 
> > ...shouldn't this mention 'PCI' somewhere? This feature does not make
> > sense on any transport but PCI.  
> 
> Yeah. When Michael suggested this feature, he also said:
> 
> https://patchwork.kernel.org/patch/10285541/
> """
> It seems PCI specific so non pci transports would disable the feature
> for now.
> """
> 
> Based on the current description, transports other than
> PCI would have this feature bit disabled.
> 
> Do you think there is any possibility that other transports
> may reuse this feature bit to do something similar?

I don't know about mmio (and I'm not sure how actively it is
developed), but ccw is so different that I don't see how it could match
to anything there.

> 
> Or do you think we should make this feature bit PCI specific
> and don't leave any chance for other transports to reuse
> it, e.g. by naming it as VIRTIO_F_PCI_SR_IOV?
> 
> Or do you think it's OK to still name it as VIRTIO_F_SR_IOV,
> but add the word 'PCI' when talking about the capability,
> e.g.:
> 
> A device SHOULD offer VIRTIO_F_SR_IOV if it presents a PCI
> SR-IOV capability structure.  A device MAY fail to operate
> further if VIRTIO_F_SR_IOV is not accepted.

Adding 'PCI' to the description is probably best, I think.

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