[virtio-dev] Re: [PATCH v2] content: document SR-IOV driver requirements

[Cornelia Huck <cohuck@redhat.com>]

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?

> +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?

It might be helpful to add some explanatory text outside of the
conformance statement so we don't stumble over this in the future.

> +
>  \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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