[virtio-comment] Re: [PATCH v2 1/4] Add virtio Admin Virtqueue

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 1/28/2022 2:14 PM, Cornelia Huck wrote:
> On Mon, Jan 24 2022, Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
>
>> In one of the many use cases a user wants to manipulate features and
>> configuration of the virtio devices regardless of the device type
>> (net/block/console). Some of this configuration is generic enough. i.e
>> Number of MSI-X vectors of a virtio PCI VF device. There is a need to do
>> such features query and manipulation by its parent PCI PF.
>>
>> Currently virtio specification defines control virtqueue to manipulate
>> features and configuration of the device it operates on. However,
>> control virtqueue commands are device type specific, which makes it very
>> difficult to extend for device agnostic commands.
>>
>> To support this requirement in elegant way, this patch introduces a new
>> admin virtqueue. Admin virtqueue will use the same command format for all
>> types of virtio devices.
>>
>> Manipulate features via admin virtqueue is asynchronous, scalable, easy
>> to extend and doesn't require additional and expensive on-die resources
>> to be allocated for every new feature that will be added in the future.
>>
>> Subsequent patches make use of this admin virtqueue.
>>
>> Reviewed-by: Parav Pandit <parav@nvidia.com>
>> Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
>> ---
>>   admin-virtq.tex | 89 +++++++++++++++++++++++++++++++++++++++++++++++++
>>   content.tex     |  9 +++--
>>   2 files changed, 96 insertions(+), 2 deletions(-)
>>   create mode 100644 admin-virtq.tex
>>
>> diff --git a/admin-virtq.tex b/admin-virtq.tex
>> new file mode 100644
>> index 0000000..1a41c22
>> --- /dev/null
>> +++ b/admin-virtq.tex
>> @@ -0,0 +1,89 @@
>> +\section{Admin Virtqueues}\label{sec:Basic Facilities of a Virtio Device / Admin Virtqueues}
>> +
>> +Admin virtqueue is used to send administrative commands to manipulate
>> +various features of the device and/or to manipulate various features,
>> +if possible, of another device within the same group (e.g. PCI VFs of
>> +a parent PCI PF device are grouped together. These devices can be
>> +optionally managed by its parent PCI PF using its admin virtqueue.).
>> +
>> +Use of Admin virtqueue is negotiated by the VIRTIO_F_ADMIN_VQ
>> +feature bit.
>> +
>> +Admin virtqueue index may vary among different device types.
> So, my understanding is:
> - any device type may or may not support the admin vq
> - if the device type wants to be able to accommodate the admin vq, it
>    also needs to specify where it shows up when the feature is negotiated
>
> Do we expect that eventually all device types will need to support the
> admin vq (if some use case comes along that will require all devices to
> participate, for example?)
>
> Anyway, I'd reword the two sentences above:
>
> "An admin virtqueue exists for a certain device if VIRTIO_F_ADMIN_VQ is
> negotiated. The index of the admin virtqueue is device type specific."

Ok. I'll use this in V3, unless we'll find some other mechanism that is 
not so clear now.

>
> (...)
>
>> @@ -6847,6 +6849,9 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
>>     that the driver can reset a queue individually.
>>     See \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}.
>>   
>> +  \item[VIRTIO_F_ADMIN_VQ (41)] This feature indicates that
>> +  the device supports administration virtqueue negotiation.
> Maybe
>
> "This feature indicates that an administration virtqueue is supported." ?

Ok.


>
>> +
>>   \end{description}
>>   
>>   \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}

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