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

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Note: the page above is unfortunately out of date and out of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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