Re: [virtio] [PATCH] virtqueue: flexible layout, size, alignment

[Stefan Hajnoczi <stefanha@redhat.com>]

On Wed, Sep 11, 2013 at 05:46:39PM +0300, Michael S. Tsirkin wrote:
> diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
> index fcd9fd7..9a40973 100644
> --- a/virtio-v1.0-wd01-part1-specification.txt
> +++ b/virtio-v1.0-wd01-part1-specification.txt
> @@ -223,6 +223,45 @@ transmit and one for receive.  Each queue has a 16-bit queue size
>  parameter, which sets the number of entries and implies the total size
>  of the queue.
>  
> +Each virtqueue consists of three parts:
> +
> +	Descriptor Table
> +	Available Ring
> +	Used Ring
> +
> +where each part is physically-contiguous in guest memory,
> +and has different alignment requirements.
> +
> +The Queue Size field controls the total number of bytes
> +required for each part of the virtqueue.

I'm lost here because I don't know where the Queue Size field comes
from.  I think the table needs to be shown first, then the explanation
can be given:

...table...

The Size column gives the total number of bytes required for each part
of the virtqueue.

> +
> +The memory aligment and size requirements, in bytes, of each part of the
> +virtqueue are summarized in the following table (qsz is the Queue Size field):

Perhaps it's clearer to say:

qsz is the maximum number of buffers.  For example, if qsz is 4 then at
most 4 buffers can be transferred at any given time.

> @@ -814,9 +852,11 @@ This is done as follows, for each virtqueue a device has:
>    always a power of 2. This controls how big the virtqueue is
>    (see "2.1.4. Virtqueues"). If this field is 0, the virtqueue does not exist.
>  
> -3. Allocate and zero virtqueue in contiguous physical memory, on
> -  a 4096 byte alignment. Write the physical address, divided by
> -  4096 to the Queue Address field.[6]
> +3. Optionally, select a smaller virtqueue size and write it in the Queue Size
> +   field.

3...

> +3. Allocate and zero Descriptor Table, Available and Used rings for the
> +   virtqueue in contiguous physical memory.

...and 3.  Should this list be renumbered?