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

[Lars Ganrot <lga@napatech.com>]

Hi Michael,

I'm trying to understand your Sep 28 13:32 example (text copied below). I've in-lined my questions [#lga#].

>
> 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

[#lga#] Is index 0 above a typo (makes more sense if it is 1)?

> 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

[#lga#] Should above line be: "DESC[0].flags =  DESC[1].flags  & DESC_WRAP"?

>
> 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.

[#lga#] At this point, has the device returned both buffers *(1234) and *(5678) to the driver?
[#lga#] If so, how would out-of-order completion avoid head of line blocking?
[#lga#] E.g. the device is done with id=1 and *(5678), but not id=0 and *(1234).

>
> 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

[#lga#] Index typo in all 3 lines above? DESC[1] makes more sense.

>
> 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.

Best Regards,

-Lars
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