On Fri, Feb 22, 2013 at 10:32:46AM +1030, Rusty Russell wrote:

"Michael S. Tsirkin" [off-list ref] writes:

quoted

On Tue, Feb 19, 2013 at 06:26:26PM +1030, Rusty Russell wrote:

quoted

These are specialized versions of virtqueue_add_buf(), which cover
over 50% of cases and are far clearer.

In particular, the scatterlists passed to these functions don't have
to be clean (ie. we ignore end markers).

FIXME: I'm not sure about the unclean sglist bit.  I had a more
ambitious one which conditionally ignored end markers in the iterator,
but it was ugly and I suspect this is just as fast.  Maybe we should
just fix all the drivers?

Signed-off-by: Rusty Russell <redacted>

Looking at code, it seems that most users really have a single sg, in
low memory. So how about simply passing void * instead of sg? Whoever
has multiple sgs can use the rich interface.

Good point, let's do that:
1) Make virtqueue_add_outbuf()/inbuf() take a void * and len.
2) Transfer users across to use that.
3) Make everyone else use clean scatterlists with virtqueue_add_sgs[].
4) Remove virtqueue_add_bufs().

quoted

Long term we might optimize this unrolling some loops, I think
I saw this giving a small performance gain for -net.

I *think* we could make virtqueue_add() an inline and implement an
virtqueue_add_outsg() wrapper and gcc will eliminate the loops for us.
But not sure it's worth the text bloat...

Cheers,
Rusty.

inline is mostly useless nowdays...  We can make it a static function and
let gcc decide.

-- 
MST