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

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

On Wed, Oct 04, 2017 at 02:39:01PM +0200, Jens Freimann wrote:
> On Sun, Oct 01, 2017 at 04:08:29AM +0000, Michael S. Tsirkin wrote:
> > On Thu, Sep 28, 2017 at 09:44:35AM +0000, Liang, Cunming wrote:
> > > 
> > > Get it now. Please correct me if I missing something.
> > > 
> > > 
> > > Flags status hints,
> > > 
> > > - DESC_DRIVER only: driver owns the descriptor w/o available info ready for device to use
> > > 
> > > - DESC_DRIVER | DESC_WRAP: driver has prepared an available descriptor, device hasn't used it yet
> > > 
> > > - None: device has used the descriptor, and write descriptor out
> > > 
> > > - DESC_WRAP only: shall not happen, device make sure to clear it
> > > 
> > > 
> > > Polling behavior is,
> > > 
> > > - Device monitor DESC_WRAP bit set or not; If set, go to use descriptor and clear DESC_DRIVER bit in the end (note: always need to clear DESC_WRAP)
> > > 
> > > - Driver monitor DESC_DRIVER bit cleared or not; If cleared, reclaim descriptor(set DESC_DRIVER) and set DESC_WRAP once new available descriptor get ready to go
> > > 
> > > 
> > > --
> > > Steve
> > 
> > 
> > Hmm no, not what I had in mind.
> > 
> > DESC_DRIVER: used by driver to poll. Driver sets it when writing a
> > descriptor.  Device clears it when overwriting a descriptor.
> > Thus driver uses DESC_DRIVER to detect that device data in
> > descriptor is valid.
> 
> Basically DESC_HW from v2 split in two?

Yes in order to avoid overwriting all descriptors.

> > 
> > DESC_WRAP: used by device to poll. Driver sets it to a *different*
> > value every time it overwrites a descriptor. How to achieve it?
> > since descriptors are written out in ring order,
> > simply maintain the current value internally (start value 1) and flip it
> > every time you overwrite the first descriptor.
> > Device leaves it intact when overwriting a descriptor.
> 
> This is confusing me a bit.
> 
> My understanding is: 1. the internally kept wrap value only flipped when the
> first
> descriptor is overwritten
> 
> 2. the moment the first descriptor is written the internal wrap value
> is flipped 0->1 or 1->0 and this value is written to every descriptor
> DESC_WRAP until we reach the first descriptor again

Yes this is what I tried to say. Can you suggest a better wording then?

> > 
> > After writing down this explanation, I think the names aren't
> > great.
> > 
> > Let me try an alternative explanation.
> > 
> > ---------------
> > A two-bit field, DRIVER_OWNER, signals the buffer ownership.
> > It has 4 possible values:
> > values 0x1, 0x11 are written by driver
> > values 0x0, 0x10 are written by device
> 
> The 0x prefix might add to the confusion here. It is really just two
> bits, no?

Ouch. Yes I meant 0b. Thanks!

> > each time driver writes out a descriptor, it must make sure
> > that the high bit in OWNER changes.
> > 
> > each time device writes out a descriptor, it must make sure
> > that the high bit in OWNER does not change.
> > 
> > this is exactly the same functionally, DRIVER is high bit and
> > WRAP is the low bit.  Does this make things clearer?
> 
> So far it makes sense to me.
> > ---------------
> > 
> > 
> > 
> > Maybe the difference between device and driver
> > is confusing. We can fix that by changing the values.
> > Here is an alternative. Let me know if you like it better -
> > I need to think a bit more to make sure it works,
> > but to give you an idea:
> > 
> > 
> > ---------------
> > A two-bit field, DRIVER_OWNER, signals the buffer ownership.
> > It has 4 possible values:
> > values 0x1, 0x10 are written by driver
> > values 0x0, 0x11 are written by device
> > 
> > each time driver writes out a descriptor, it must make sure
> > that the high bit in OWNER changes. Thus first time
> > it writes 0x10, next time 0x1, then 0x10 again.
> > 
> > each time device writes out a descriptor, it must make sure
> > that the low bit in OWNER changes.  Thus first time
> > it writes 0x11, next time 0x0, then 0x11 again.
> 
> DESC_WRAP is changed by the device now, so this would work differently
> than in the scenario from above. This would mean we don't need the
> internally kept wrap value, right?

I think no but I did not complete simulating this yet so I don't
yet know if it even works.

> 
> regards,
> Jens

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