Re: [virtio-dev] Re: [PATCH v1] content: support SR-IOV

[Tiwei Bie <tiwei.bie@intel.com>]

On Tue, May 22, 2018 at 01:32:29AM +0300, Michael S. Tsirkin wrote:
> On Mon, May 21, 2018 at 06:52:06PM +0800, Tiwei Bie wrote:
> > Reserve a feature bit for virtio devices which support SR-IOV.
> 
> s/Reserve/Allocate/ in that you define the behaviour here.

Got it. Thanks!

> 
> > Suggested-by: Michael S. Tsirkin <mst@redhat.com>
> > Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
> 
> Pls go ahead and add a github issue, you can then
> include a Fixes: tag with a link to it,
> which comes it handy down the road for voting.

Got it. I will send a new version to address this.

> 
> > ---
> > More details can be found from this thread:
> > 
> > https://patchwork.kernel.org/patch/10285541/
> > 
> > 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 7a92cb1..9f981fb 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 +5348,9 @@ 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.
> > +  Currently only virtio PCI devices support this feature.
> >  \end{description}
> >  
> >  \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> > @@ -5363,6 +5366,10 @@ addresses to the device.
> >  
> >  A driver SHOULD accept VIRTIO_F_RING_PACKED if it is offered.
> >  
> > +A driver SHOULD accept VIRTIO_F_SR_IOV if it is offered.
> > +If VIRTIO_F_SR_IOV has been negotiated, a driver can
> > +access device's PCI SR-IOV capability structure.
> 
> Does "access" here really mean "enable virtual functions"
> through the PCI SR-IOV capability?

Hmm.. I'll make it more explicit in the next version,
something like:

If VIRTIO_F_SR_IOV has been negotiated, a driver can
enable virtual functions through 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
> > @@ -5376,6 +5383,10 @@ accepted.
> >  If VIRTIO_F_IN_ORDER has been negotiated, a device MUST use
> >  buffers in the same order in which they have been available.
> >  
> > +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.
> 
> Why is the last sentence here? What are you trying to accomplish?  It
> seems that if we allow drivers not to accept the bit, we should require
> the devices to function without it or at least not go out of our way to
> say they do not have to.

The last sentence "A device MAY fail to operate further
if VIRTIO_F_SR_IOV is not accepted" means if the driver
doesn't accept this feature, the device may fail to work,
e.g. setting VIRTIO_CONFIG_S_FEATURES_OK may fail. And
in this case (i.e. the device fails to work), it will be
necessary for driver to accept this feature in order to
work with this device.

So it's not to allow drivers not to accept the feature,
instead, it just tells device's behaviour (i.e. device
may fail to work) if this feature isn't accepted. And
it leaves the choice to device's implementation.

It's similar to the description for the IOMMU_PLATFORM
feature:

"""
A device SHOULD offer VIRTIO_F_IOMMU_PLATFORM if it is behind an IOMMU that
translates bus addresses from the device into physical addresses in memory.
A device MAY fail to operate further if VIRTIO_F_IOMMU_PLATFORM is not
accepted.
"""

Thanks for the review!

Best regards,
Tiwei Bie

> 
> 
> > +
> >  \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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