[virtio-dev] Re: [virtio-comment] [PATCH v14 04/11] transport-pci: Avoid first vq index reference

[Halil Pasic <pasic@linux.ibm.com>]

On Wed, 19 Apr 2023 04:46:32 +0300
Parav Pandit <parav@nvidia.com> wrote:

> Drop reference to first virtqueue as it is already
> covered now by the generic section in first patch.
> 

TL;DR:

Acked-by: Halil Pasic <pasic@linux.ibm.com>

Rationale: 
---------
I agree, what a virtqueue index is should be described in a
single place instead all over the place. And further improvements can be done
on top of this series. 

Discussion:
-----------
Is it indeed covered now by the generic section in first patch?

In [1] you state the following: "There is nothing like a zero based
index. A VQ can be any u16 _number_ in range of 0 to 65534.
Number can also start from zero. So zero based index = zero based
number."

I don't really understand what do you mean by this, but I'm afraid
it is not consistent with my understanding. Can not be any number in
range of 0 to 65534.

Rather in general it depends on the device type, and on the device
how many queues it support. Let us call this number dev_vq_max. And
if dev_vq_max > N > 0 index is associated with a queue the N-1 index is
also associated with a queue.

Let me also note that the number of queues supported/provided by the
device is a matter of the device. For many devices the number of queues
is a constant given in the specification. A Network Device tells
its driver how may queues it supports via max_virtqueue_pairs,
Crypto Device via max_dataqueues, and Console Device via max_dataqueues.
The transports PCI also has num_queues which is documented as "The
device specifies the maximum number of virtqueues supported here." while
CCW and MMIO have no transport specific means to tell the driver about
the number of queues supported by the device. In any case, if the number
of queues provided by the device is N, each of those queues is uniquely
identified by exactly one index from the range [0..N-1]. Furthermore
the role a certain queue plays is determined by its index. E.g. for
the Console Device virtqueue identified by the index 3 is the "control
receiveq".

[1]
https://lore.kernel.org/virtio-comment/d62d2090-de0d-0ea6-5c85-ecf19ae828c7@nvidia.com/

[..]
>  
>  \begin{enumerate}
> -\item Write the virtqueue index (first queue is 0) to \field{queue_select}.
> +\item Write the virtqueue index to \field{queue_select}.
>  
>  \item Read the virtqueue size from \field{queue_size}. This controls how big the virtqueue is
>    (see \ref{sec:Basic Facilities of a Virtio Device / Virtqueues}~\nameref{sec:Basic Facilities of a Virtio Device / Virtqueues}). If this field is 0, the virtqueue does not exist.


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