Re: [virtio-dev] [PATCH v3] content: support SR-IOV

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

On Wed, May 23, 2018 at 08:54:47PM +0300, Michael S. Tsirkin wrote:
> On Tue, May 22, 2018 at 06:26:15PM +0800, Tiwei Bie wrote:
> > Allocate a feature bit for virtio devices which support SR-IOV.
> > 
> > Suggested-by: Michael S. Tsirkin <mst@redhat.com>
> > Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
> > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/11
> > ---
> > More details can be found from this thread:
> > https://patchwork.kernel.org/patch/10285541/
> > 
> > This patch needs below patch applied first:
> > https://github.com/oasis-tcs/virtio-spec/issues/10
> > https://lists.oasis-open.org/archives/virtio-dev/201805/msg00046.html
> > 
> > v2 -> v3:
> > - Improve the wording (Cornelia);
> > 
> > v1 -> v2:
> > - s/Reserve/Allocate/ (MST);
> > - Add a Fixes tag (MST);
> > - Be more explicit in driver requirement (MST);
> > - Remove the "device MAY fail" description (MST);
> > - Rebase on IO_BARRIER patch;
> > 
> > RFC -> v1:
> > - Mention PCI in the description (Cornelia);
> > 
> >  content.tex | 15 +++++++++++++--
> >  1 file changed, 13 insertions(+), 2 deletions(-)
> > 
> > diff --git a/content.tex b/content.tex
> > index 95c243f..e9e6f9a 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 36] Feature bits reserved for extensions to the queue and
> > +\item[24 to 37] Feature bits reserved for extensions to the queue and
> >    feature negotiation mechanisms
> >  
> > -\item[37 and above] Feature bits reserved for future extensions.
> > +\item[38 and above] Feature bits reserved for future extensions.
> >  \end{description}
> >  
> >  \begin{note}
> > @@ -5357,6 +5357,9 @@ Descriptors} and \ref{sec:Packed Virtqueues / Indirect Flag: Scatter-Gather Supp
> >    better performance.  This feature indicates whether
> >    a stronger form of barrier suitable for hardware
> >    devices is necessary.
> > +  \item[VIRTIO_F_SR_IOV(37)] This feature indicates that
> > +  the device supports Single Root I/O Virtualization.
> > +  Currently only PCI devices support this feature.
> 
> I guess the assumption is that all VFs and the PF are of the same type?
> 
> I feel it might be handy down the road to support mixing
> types. For this reason, to avoid binding a wrong driver
> to a VF, I propose that all VFs have this bit too,
> and require that drivers ignore VFs without this bit.
> 
> What do you think?

Thinking more about it, I can see how this might
interfere with passing VFs through to legacy nested guests.
How about reversing it then?

Require that drivers MUST NOT negotiate VIRTIO_F_SR_IOV
if device does not have an SRIOV capability or
is not a PCI device, in particular a VF.

And say a device without SRIOV cap SHOULD NOT expose this bit.



> >  \end{description}
> >  
> >  \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> > @@ -5376,6 +5379,11 @@ A driver SHOULD accept VIRTIO_F_IO_BARRIER if it is offered.
> >  If VIRTIO_F_IO_BARRIER has been negotiated, a driver MUST use
> >  the barriers suitable for hardware devices.
> >  
> > +A driver SHOULD accept VIRTIO_F_SR_IOV if it is offered.
> > +If VIRTIO_F_SR_IOV has been negotiated, a driver can
> > +enable virtual functions through the device's PCI SR-IOV
> > +capability structure.
> 
> I feel the last sentence isn't clear enough.  How about
> 
> a driver MUST negotiate VIRTIO_F_SR_IOV and complete the feature
> negotiation (including setting the DRIVER_OK \field{status} bit) before
> enabling virtual functions through the device's PCI SR-IOV capability
> structure.
> 
> > +
> >  \devicenormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> >  
> >  A device MUST offer VIRTIO_F_VERSION_1.  A device MAY fail to operate further
> > @@ -5392,6 +5400,9 @@ buffers in the same order in which they have been available.
> >  A device MAY fail to operate further if VIRTIO_F_IO_BARRIER
> >  is not accepted.
> >  
> > +A device SHOULD offer VIRTIO_F_SR_IOV if it presents a PCI
> > +SR-IOV capability structure.
> > +
> >  \section{Legacy Interface: Reserved Feature Bits}\label{sec:Reserved Feature Bits / Legacy Interface: Reserved Feature Bits}
> >  
> >  Transitional devices MAY offer the following:
> 
> 
> 
> > -- 
> > 2.17.0
> > 
> > 
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