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. Thanks. /mjt, who lost 2 more days and had another sleepless night trying to understand what's going wrong... -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
