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

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 2/2/2022 2:21 PM, Cornelia Huck wrote:
> On Wed, Feb 02 2022, Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
>
>> 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.
> For your purpose, it would probably be best to simply go with 43 in your
> next update.

it won't align with the Feature bits section so I'll keep it 41 for v3 
and hope it will be resolved by v4...

>
> We have to come to a conclusion before we merge any new feature using a
> bit in that range anyway... and I really would like to have that cleared
> up before we do our draft :/
>