Re: [PATCH] Reserve more feature bits for device type usage

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 2/2/2022 1:52 PM, Cornelia Huck wrote:
> On Wed, Feb 02 2022, Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
>
> > On 1/14/2022 1:12 PM, Cornelia Huck wrote:
> > > Feature bits 41 and above are noted as being reserved for future
> > > extensions. However, the net device has been using bits in that space
> > > for some time now, as it already used up the device type specific
> > > range up to 23.
> > >
> > > To avoid problems in the future, let's designate bits 50 to 127 to
> > > device type specific usage (which accommodates current usage by the
> > > net driver, and gives breathing room for future type specific bits),
> > > and declare bits 41 to 49 and bits 128 and above to be reserved for
> > > future extensions (which gives us some time before bit numbers move
> > > beyond 63, which would need some changes in existing device and driver
> > > implementations.)
> > >
> > > Reported-by: Max Gurtovoy <mgurtovoy@nvidia.com>
> > > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/131
> > > Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> > > ---
> > >    content.tex | 4 ++--
> > >    1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/content.tex b/content.tex
> > > index 32de6685c50b..c6f116c7aa39 100644
> > > --- a/content.tex
> > > +++ b/content.tex
> > > @@ -97,12 +97,12 @@ \section{Feature Bits}\label{sec:Basic Facilities of a Virtio Device / Feature B
> > >    Feature bits are allocated as follows:
> > >    
> > >    \begin{description}
> > > -\item[0 to 23] Feature bits for the specific device type
> > > +\item[0 to 23, and 50 to 127] Feature bits for the specific device type
> > >    
> > >    \item[24 to 40] Feature bits reserved for extensions to the queue and
> > >      feature negotiation mechanisms
> > >    
> > > -\item[41 and above] Feature bits reserved for future extensions.
> > > +\item[41 to 49, and 128 and above] Feature bits reserved for future extensions.
> > >    \end{description}
> > >    
> > >    \begin{note}
> > Cornelia,
> >
> > I've noticed that Legacy net device has VIRTIO_NET_F_GUEST_RSC4 (41) and
> > VIRTIO_NET_F_GUEST_RSC6 (42).
> >
> > Do we need to reserved them too or keep the fix as-is ?
> Eww. I think those bits shouldn't have been hiding in the "legacy"
> section, as their higher number indicates that they are not for a legacy
> device in the spec sense.
>
> They have been introduced in
> https://github.com/oasis-tcs/virtio-spec/issues/21 in 2018, I'm
> wondering how old those Windows drivers referred to in there are. IOW,
> would a new feature bit 41/42 break things for currently used Windows
> drivers if it showed up on a virtio-net device? If we still care about
> those drivers, we should reserve bits 41 and 42 as do-not-use, I guess.

I've not idea about the windows drivers.

But the bit 41 is currently the bit I use to negotiate the AQ in the 
last patchset so we need to decide how to progress.

Thanks.