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

["Liang, Cunming" <cunming.liang@intel.com>]

Hi Michael,

> -----Original Message-----
> From: virtio-dev@lists.oasis-open.org [mailto:virtio-dev@lists.oasis-open.org]
> On Behalf Of Michael S. Tsirkin
> Sent: Sunday, September 10, 2017 1:06 PM
> To: virtio-dev@lists.oasis-open.org
> Cc: virtualization@lists.linux-foundation.org
> Subject: [virtio-dev] packed ring layout proposal v3
> 
[...]
> * Descriptor ring:
> 
> Driver writes descriptors with unique index values and DESC_DRIVER set in
> flags.
> Descriptors are written in a ring order: from start to end of ring, wrapping
> around to the beginning.
> Device writes used descriptors with correct len, index, and DESC_HW clear.
> Again descriptors are written in ring order. This might not be the same order
> of driver descriptors, and not all descriptors have to be written out.
> 
> Driver and device are expected to maintain (internally) a wrap-around bit,
> starting at 0 and changing value each time they start writing out descriptors
> at the beginning of the ring. This bit is passed as DESC_WRAP bit in the flags
> field.

One simple question there, trying to understand the usage of DESC_WRAP flag.

DESC_WRAP bit is a new flag since v2. It's used to address 'non power-of-2 ring sizes' mentioned in v2?

Being confused by the statement of wrap-around bit here, it's an internal wrap-around counter represented by single bit (0/1)?
DESC_WRAP can appear on any descriptor entry in the ring, why it highlights changing value at the beginning of the ring?

> 
> Flags are always set/cleared last.
> 
> Note that driver can write descriptors out in any order, but device will not
> execute descriptor X+1 until descriptor X has been read as valid.
> 
> Driver operation:
> 
[...]
> 
> DESC_WRAP - device uses this field to detect descriptor change by driver.

Device uses this field to detect change of wrap-around boundary by driver? 

[...]
> 
> Device operation (using descriptors):
> 
[...]
> 
> DESC_WRAP - driver uses this field to detect descriptor change by device.

Driver uses this field to detect change of wrap-around boundary by device?

By using this, driver doesn't need to maintain any internal wrap-around count, but being aware of wrap-around by DESC_WRAP flag.


Thanks,
Steve

> 
[...]
> 
> ---
> 
> Note: should this proposal be accepted and approved, one or more
>       claims disclosed to the TC admin and listed on the Virtio TC
>       IPR page https://www.oasis-open.org/committees/virtio/ipr.php
>       might become Essential Claims.
> Note: the page above is unfortunately out of date and out of
>       my hands. I'm in the process of updating ipr disclosures
>       in github instead.  Will make sure all is in place before
>       this proposal is put to vote. As usual this TC operates under the
>       Non-Assertion Mode of the OASIS IPR Policy, which protects
>       anyone implementing the virtio spec.
> 
> --
> MST
> 
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