RE: [kvm-devel] [PATCH 00/10] PV-IO v3

Dor Laor
2007-08-20T14:03:30Z

>> > 2) We either need huge descriptors or some chaining
>mechanism to
>> > handle scatter-gather.
>> >
>>
>> Or, my preference, have a small sglist in the descriptor;
>
>
>Define "small" ;)
>
>There a certainly patterns that cannot/will-not take advantage of SG
>(for instance, your typical network rx path), and therefore the sg
>entries are wasted in some cases. Since they need to be (IMHO) u64,
>they suck down at least 8 bytes a piece. Because of this I elected to
>use the model of one pointer per descriptor, with an external
descriptor
>for SG. What are your thoughts on this?

Using Rusty's code there is no waste.
Each descriptor has a flag (head|next). Next flag stands for pointer to
the
next descriptor with u32 next index. So the waste is 4 bytes.
Sg descriptors are chained on the same descriptor ring.