netdev - Re: [PATCH 0/3] virtio-net: inline header support

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Message-ID: <20121008204145.GA17820@redhat.com>
Date:	Mon, 8 Oct 2012 22:41:45 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Thomas Lendacky <tahm@...ux.vnet.ibm.com>,
	Sasha Levin <levinsasha928@...il.com>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, avi@...hat.com, kvm@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH 0/3] virtio-net: inline header support

On Wed, Oct 03, 2012 at 04:14:17PM +0930, Rusty Russell wrote:
> "Michael S. Tsirkin" <mst@...hat.com> writes:
> 
> > Thinking about Sasha's patches, we can reduce ring usage
> > for virtio net small packets dramatically if we put
> > virtio net header inline with the data.
> > This can be done for free in case guest net stack allocated
> > extra head room for the packet, and I don't see
> > why would this have any downsides.
> 
> I've been wanting to do this for the longest time... but...
> 
> > Even though with my recent patches qemu
> > no longer requires header to be the first s/g element,
> > we need a new feature bit to detect this.
> > A trivial qemu patch will be sent separately.
> 
> There's a reason I haven't done this.  I really, really dislike "my
> implemention isn't broken" feature bits.  We could have an infinite
> number of them, for each bug in each device.
> 
> So my plan was to tie this assumption to the new PCI layout.

I don't object but old qemu has this limitation for s390 as well,
and that's not using PCI, right? So how do we detect
new hypervisor there?

-- 
MST
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