[Qemu-devel] Re: persistent tun & different virtual NICs & dead guest network

[Avi Kivity <avi@redhat.com>]

(cc qemu-devel)

Michael Tokarev wrote:
> Hello.
>
> 2 days debugging an.. issue here, and finally got it.
> To make the long and painful (it was for me anyway)
> story short...
>
> kvm provides a way to control various offload settings
> on the "host side" of the tun network device (I mean
> the `-net tap' setup) from within guest.  I.e., guest
> can set/clear various offload bits according to its
> capabilities/wishes.
>
> The problem is that different virtual NICs as used by
> kvm/qemu expects and sets different offload bits for
> the virtual NIC.  And sets only those bits which -
> as they "think" - differs from the default (all-off).
>
> This means that when changing virtual NIC model AND
> using persistent tun device, it's very likely to get
> inconsistent flags.
>
> For example, here's how the offload settings on the
> host looks like after using e1000 driver in guest
> (freshly created persistent tun device):
>
>  rx-checksumming: on
>  tx-checksumming: on
>  scatter-gather: on
>  tcp segmentation offload: on
>  udp fragmentation offload: off
>  generic segmentation offload: off
>  large receive offload: off
>
> Here's the same setting when using virtio_net
> instead:
>
>  rx-checksumming: on
>  tx-checksumming: off
>  scatter-gather: off
>  tcp segmentation offload: off
>  udp fragmentation offload: off
>  generic segmentation offload: off
>  large receive offload: off
>
> I.e., only rx-checksumming.  When using virtio_net
> from 2.6.29, which supports LRO, it also turns on
> large receive offload.
>
> Now, say, I tried a host with e1000 driver, and it
> turned on tx, sg and tso bits.  And now I'm trying
> to run a guest with new virtio-net NIC instead.  It
> turns on lro bit, but the network does not work anyway:
> almost any packet that's being sent from host to the
> guest has incorrect checksum - because the NIC is marked
> as able to do tx-checksumming but it does not do it.
> The network is dead.
>
> Now, after trying that and this, not understanding
> what's going on etc, let's reboot back with e1000
> NIC which worked a few minutes ago... just to discover
> that it does not work anymore too!  Because previous
> attempt with virtio_net resulted in lro being on, but
> the driver does not support it!  So now, we've non-
> working network again, and now, it does not matter
> which driver we'll try: neither of them will work
> because the offload settings are broken.
>
> It's more: one can't control this stuff from the
> host side using standard ethtool: it says that
> the operation is not supported (I wonder how kvm
> performs the settings changes).
>
> The solution here is to re-create the tun device
> before changing the virtual NIC model.  But it
> isn't always possible, esp. when guests are
> being run from non-root user (where persistent
> tun devices are most useful).
>
> Can this be fixed somehow please?
>
> I think all the settings should be reset to 0
> when opening the tun device.

This should definitely be fixed.  I'll look at writing a patch.

-- 
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