netdev - Re: [PATCH 0/2] fix kernel crash with macvtap on top of LRO

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Message-ID: <1371048996.1894.15.camel@bwh-desktop.uk.level5networks.com>
Date:	Wed, 12 Jun 2013 15:56:36 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	David Miller <davem@...emloft.net>, <eric.dumazet@...il.com>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<rusty@...tcorp.com.au>,
	<virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH 0/2] fix kernel crash with macvtap on top of LRO

On Mon, 2013-06-10 at 10:07 +0300, Michael S. Tsirkin wrote:
> On Thu, Feb 07, 2013 at 01:14:20PM -0500, David Miller wrote:
> > From: Ben Hutchings <bhutchings@...arflare.com>
> > Date: Thu, 7 Feb 2013 16:20:46 +0000
> > 
> > > If the consensus is still that we must preserve packets exactly (aside
> > > from the usual modifications by IP routers) then LRO should be disabled
> > > on all devices for which forwarding is enabled.
> > 
> > I believe this is still undoubtedly the consensus.
> 
> With virtio we are getting packets from a linux host,
> so we could thinkably preserve packets exactly
> even with LRO. I am guessing other hardware could be
> doing this as well.
> 
> I am not sure what information would need to be preserved -
> could someone help clarify please?

Some LRO implementations may not preserve:

- Packet boundaries
  - TSO/GSO produces packets all the same size, except possibly for the
    last one.  GRO therefore flushes a flow after merging a packet with
    a different segment size.
- IPv4 TTL, IPv6 hop-limit, TCP timestamp
  - TSO/GSO will put the same values in all packets.  GRO flushes a flow
    if they change.
- IPv4 fragment ID
  - TSO/GSO produces consecutive fragment IDs.  GRO flushes a flow
    if it sees a non-consecutive fragment ID.
- MAC header, IPv4 TOS, IPv6 traffic class
  - Should be the same for all packets in a flow.  GRO actually checks
    and flushes a flow if they change.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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