Re: [virtio-comment] Re: [virtio] [PATCH] virtio: clarify feature negotiation

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

On Thu, Apr 07, 2022 at 04:28:03PM +0200, Cornelia Huck wrote:
> [sorry about resurrecting an undead thread; I'm trying to collect some
> editorial changes]
> 
> On Wed, Jan 26 2022, Jean-Philippe Brucker <jean-philippe@linaro.org> wrote:
> 
> > On Mon, Jan 24, 2022 at 04:42:35PM -0500, Michael S. Tsirkin wrote:
> >> So from that point of view, we are actually relaxing the requirements,
> >> and yes we'll need to carefully go over each instance of
> >> "offered".
> >> I thought I did, but now I did a quick grep again and I found instances
> >> in virtio-iommu.tex and virtio-gpio.tex
> >> Both of these are new, so I think we can just fix them to "negotiated".
> >> 
> >> Jean-Philippe, Viresh, what do you think? Would it be a problem to
> >> replace offered with negotiated, and restrict iommu and gpio drivers to
> >> access config space after FEATURES_OK?
> >
> > No problem for iommu. The config space fields domain_range and input_range
> > are not useful to the driver before DRIVER_OK. Reading field bypass
> > earlier could be useful for debugging, but in that case going a few more
> > steps into device initialization wouldn't hurt.
> 
> For domain_range and input_range, offered->negotiated looks like the
> right thing to do. However, I'm a bit unsure about bypass. We currently
> have:
> 
> "
> An endpoint is in bypass mode if:
> \begin{itemize}
>   \item the VIRTIO_IOMMU_F_BYPASS_CONFIG feature is offered and:
>     \begin{itemize}
>       \item config field \field{bypass} is 1 and the endpoint is
>         not attached to a domain. This applies even if the driver
>         does not accept the VIRTIO_IOMMU_F_BYPASS_CONFIG feature
>         and the device initializes field \field{bypass} to 1.
> 
>         or
>       \item the endpoint is attached to a domain with
>         VIRTIO_IOMMU_ATTACH_F_BYPASS.
>     \end{itemize}
> "
> 
> The "This applies even if..." clause refers to "offered, but not
> accepted". I assume we want to do offered->negotiated even here, but
> wanted to double-check.

Negotiated is same as accepted, I think it's good as is

-- 
MST