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.