Subject: Re: [PATCH 2/3] virtio-net: correct capacity math on ring full

From: Rusty Russell <[email-masked]>
Date: Thu, 04 Oct 2012 09:54:01 +0930

"Michael S. Tsirkin" <[email-masked]> writes:
> Capacity math on ring full is wrong: we are
> looking at num_sg but that might be optimistic
> because of indirect buffer use.
>
> The implementation also penalizes fast path
> with extra memory accesses for the benefit of
> ring full condition handling which is slow path.
>
> It's easy to query ring capacity so let's do just that.

This path will reduce the actual queue use to worst-case assumptions.
With bufferbloat maybe that's a good thing, but it's true.

If we do this, the code is now wrong:

	/* This can happen with OOM and indirect buffers. */
	if (unlikely(capacity < 0)) {

Because this should now *never* happen.

But I do like the cleanup; returning capacity from add_buf() was always
hacky.  I've got an idea, we'll see what it looks like...

Cheers,
Rusty.