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

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

On Thu, Sep 28, 2017 at 02:49:15AM +0300, Michael S. Tsirkin wrote:
> 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

Note: I made a mistake in the email. Instead of DESC_NEXT it should read
DESC_MORE everywhere. I corrected the quoted text below for simplicity.



> 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_MORE | 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_MORE
> 
> 
> 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

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