netdev - Re: user space virtio-net exits with "truncating packet" error

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Message-ID: <4A0C2500.7090709@Voltaire.com>
Date:	Thu, 14 May 2009 17:04:48 +0300
From:	Or Gerlitz <ogerlitz@...taire.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Avi Kivity <avi@...hat.com>, netdev@...r.kernel.org,
	Gregory Haskins <ghaskins@...ell.com>,
	Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: user space virtio-net exits with "truncating packet" error

Rusty Russell wrote:
> The answer is that virtio_net by default only supports 1500 
> MTU; I've not tried larger MTUs.

Rusty,

I hoped to get some performance boost from using checksum and large-send offloads 
as an alternative to jumbo frames. Looking in the virtio-net kernel driver, I see that the probe
function checks if virtio_has_feature VIRTIO_NET_F_CSUM ... VIRTIO_NET_F_HOST_TSO ... and if yes
sets the relevant bits in the NIC features mask. Looking in the virtio qemu code, I also see some offload
related code.

In my case, both the guest and the host run 2.6.29.1 and I use ethX (igb based) -- bridge -- tapY <--> qemu configuration, where qemu is the one provided by kvm latest release (84).  Now, the virtio guest NIC doesn't expose any features (its mask being 0x20 - only highdma). Anything I can do to have offloads support for my virtio environment? 

Or.
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