Re: [virtio-dev] packed ring layout proposal v2

[Paolo Bonzini <pbonzini@redhat.com>]

On 08/02/2017 04:20, Michael S. Tsirkin wrote:
> * Scatter/gather support
> 
> We can use 1 bit to chain s/g entries in a request, same as virtio 1.0:
> 
> /* This marks a buffer as continuing via the next field. */
> #define VRING_DESC_F_NEXT       1
> 
> Unlike virtio 1.0, all descriptors must have distinct ID values.
> 
> Also unlike virtio 1.0, use of this flag will be an optional feature
> (e.g. VIRTIO_F_DESC_NEXT) so both devices and drivers can opt out of it.

I would still prefer that we had  _either_ single-direct or
multiple-indirect descriptors, i.e. no VRING_DESC_F_NEXT.  I can propose
my idea for this in a separate message.

> * Batching descriptors:
> 
> virtio 1.0 allows passing a batch of descriptors in both directions, by
> incrementing the used/avail index by values > 1.  We can support this by
> chaining a list of descriptors through a bit the flags field.
> To allow use together with s/g, a different bit will be used.
> 
> #define VRING_DESC_F_BATCH_NEXT 0x0010
> 
> Batching works for both driver and device descriptors.

I'm still not sure how this would be useful.  It cannot be mandatory to
set the bit, I think, because you don't know when the host/guest is
going to read descriptors.  So both host and guest always have to look
ahead one element in any case.

> * Non power-of-2 ring sizes
> 
> As the ring simply wraps around, there's no reason to
> require ring size to be power of two.
> It can be made a separate feature though.

Power of 2 ring sizes are required in order to ignore the high bits of
the indices.  With non-power-of-2 sizes you are forced to keep the
indices less than the ring size.  Alternatively you can do this:

> * Event index would be in the range 0 to 2 * Queue Size
> (to detect wrap arounds) and wrap to 0 after that.
> 
> The assumption is that each side maintains an internal
> descriptor counter 0 to 2 * Queue Size that wraps to 0.
> In that case, interrupt triggers when counter reaches
> the given value.

but it seems more complicated than just forcing power-of-2 and ignoring
the high bits.

Thanks,

Paolo