netdev - Re: [RFC PATCH] Regression in linux 2.6.32 virtio

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Message-Id: <200912161507.53805.rusty@rustcorp.com.au>
Date:	Wed, 16 Dec 2009 15:07:53 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	Sridhar Samudrala <sri@...ibm.com>, netdev@...r.kernel.org
Subject: Re: [RFC PATCH] Regression in linux 2.6.32 virtio_net seen with vhost-net

On Wed, 16 Dec 2009 10:02:27 am Michael S. Tsirkin wrote:
> Right. Hmm. So for this to work we'll need to
> 1. Free skb upon interrupt instead of
>    waiting for the next xmit call
> 2. Add API to query free ring capacity
> 
> Rusty, sounds like a good plan?

Well, the query stuff is not too bad, but I can't completely convince myself
it's race-free.  We don't want to do locking.

A NAPI-style solution seems cleaner, and I'm testing that now.

> We could also extend host to delay interrupt
> until there is sufficient TX capacity
> but of course we also need to support
> old hosts as well.

Xen does this, and I rejected it in favor of simple enable/disable
flags in the original virtio design.  It was probably wrong: while the
guest can enable on a "few remaining" heuristic, it's going to have
latency.  The host can do a more timely decision.

There's nothing stopping the Host from doing this heuristic today, of
course: the DISABLE flag is advisory only.  But let's check the limitations
of the guest-enable approach first?

Thanks,
Rusty.
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