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Subject: Re: [virtio-dev] Re: [PATCH v2] content: document SR-IOV driver requirements
On Fri, Jun 08, 2018 at 02:38:57PM +0200, Cornelia Huck wrote: > On Fri, 8 Jun 2018 19:14:49 +0800 > Tiwei Bie <tiwei.bie@intel.com> wrote: > > > On Fri, Jun 08, 2018 at 11:12:31AM +0200, Cornelia Huck wrote: > > > On Fri, 8 Jun 2018 10:07:01 +0800 > > > Tiwei Bie <tiwei.bie@intel.com> wrote: > > > > > > > Document the driver requirements for the VIRTIO_F_SR_IOV > > > > feature bit. > > > > > > > > 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/13 > > > > --- > > > > v2: > > > > - Fix the commit message (MST); > > > > - Improve the wording (MST); > > > > - Drop unnecessary parts (MST); > > > > > > > > content.tex | 15 +++++++++++++++ > > > > 1 file changed, 15 insertions(+) > > > > > > > > diff --git a/content.tex b/content.tex > > > > index be18234..f996fad 100644 > > > > --- a/content.tex > > > > +++ b/content.tex > > > > @@ -5387,6 +5387,21 @@ 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. > > > > > > > > +If VIRTIO_F_SR_IOV has been negotiated, a driver MAY enable > > > > +virtual functions through the device's PCI SR-IOV capability > > > > +structure. A driver MUST NOT negotiate VIRTIO_F_SR_IOV if > > > > +the device does not have a PCI SR-IOV capability structure > > > > +or is not a PCI device. A driver MUST negotiate > > > > > > Maybe I'm missing something obvious, but why should the device offer > > > the feature in the first place if it does not support the functionality? > > > > The device is not allowed to offer the feature if it > > doesn't support the functionality. > > > > From my understanding, Michael suggested this to make > > sure that in the future, the existing drivers won't > > accept this feature bit when the VF or other transport > > devices offer this feature bit, because some changes > > may be necessary for drivers to really support this > > feature offered by those devices. > > So, this is supposed to be a safety net? Ok. Yeah! > > > > > > > > > > +VIRTIO_F_SR_IOV and complete the feature negotiation > > > > +(including checking the FEATURES_OK \field{status} bit) > > > > +before enabling virtual functions through the device's > > > > +PCI SR-IOV capability structure. After once successfully > > > > +negotiating VIRTIO_F_SR_IOV, the driver MAY enable virtual > > > > +functions through the device's PCI SR-IOV capability > > > > +structure even if the device or the system has been fully > > > > +or partially reset, and even without re-negotiating > > > > +VIRTIO_F_SR_IOV after the reset. > > > > > > So, what is the general lifetime of this feature supposed to be? As > > > written here, the driver needs to negotiate the feature once and then > > > may enable virtual functions at any time in all eternity. Is this > > > intended to accommodate hardware implementations, where some kind of > > > switch is flipped once and then the functionality is available? > > > > > > Also, as the device will need to negotiate the feature at least once, > > > what is stopping it from negotiating it again in the future? Is this > > > wording intended to allow the driver to simply use virtual functions on > > > resume etc. prior to feature negotiation? > > > > This isn't intended to accommodate hardware implementations. > > > > Yes, this is intended to allow the driver to simply use virtual > > functions on resume etc. prior to feature negotiation. > > > > > > > > It might be helpful to add some explanatory text outside of the > > > conformance statement so we don't stumble over this in the future. > > > > Do you have any more specific suggestions about this? > > Sorry, not really. Maybe see the other branch of this discussion? Sure! Thanks! :) Best regards, Tiwei Bie > > > It would be quite helpful! Thanks a lot! > > > > Best regards, > > Tiwei Bie > > > > > > > > > + > > > > \devicenormative{\section}{Reserved Feature Bits}{Reserved Feature Bits} > > > > > > > > A device MUST offer VIRTIO_F_VERSION_1. A device MAY fail to operate further > > >
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