Re: [virtio-dev] packed ring layout proposal v3

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 26, 2017 at 11:38:18PM +0000, Steven Luong (sluong) wrote:
> Michael,
> 
> Would you please give an example or two how these two flags DESC_DRIVER and DESC_WRAP are used together? Like others, I am confused by the description and still don’t quite grok it.
> 
> Steven

My bad, I will need to work on it. Here is an example:

Let's assume device promised to consume packets in order

ring size = 2

Ring is 0 initialized.

Device initially polls DESC[0].flags for WRAP bit to change.

driver adds:

DESC[0].addr = 1234
DESC[0].id = 0
DESC[0].flags = DESC_DRIVER | DESC_NEXT | DESC_WRAP

and

DESC[0].addr = 5678
DESC[1].id = 1
DESC[1].flags = DESC_DRIVER | DESC_WRAP


it now starts polling DESC[0] flags.


Device reads 1234, executes it, does not use it.

Device reads 5678, executes it, and uses it:

DESC[0].id = 1
DESC[0].flags = 0

Device now polls DESC[0].flags for WRAP bit to change.

Now driver sees that DRIVER bit has been cleared, so it nows that id is
valid. I sees id 1, therefore id 0 and 1 has been read and are safe to
overwrite.

So it writes it out. It wrapped around to beginning of ring,
so it flips the WRAP bit to 0 on all descriptors now:

DESC[0].addr = 9ABC
DESC[0].id = 0
DESC[0].flags = DESC_DRIVER | DESC_NEXT


DESC[0].addr = DEF0
DESC[0].id = 1
DESC[0].flags = DESC_DRIVER


Next round wrap will be 1 again.


To summarise:

DRIVER bit is used by driver to detect device has used one or more
descriptors.  WRAP is is used by device to detect driver has made a
new descriptor available.


-- 
MST