[Qemu-devel] Re: virtio breakage with 2.6.25 guest kernel

[Anthony Liguori <anthony@codemonkey.ws>]

Alexander Graf wrote:
> Hi,
>
> while I don't fully understand the problem, here's what I experience so far:
>
> When using an openSUSE 11.0 kernel (2.6.25) in the guest, virtio on tap
> breaks with current KVM git, while it used to work before (haven't
> bisected, definitely worked in kvm-78, but is probably due to Anthony's
> rewrite). It shows the following message (comes from qemu):
>   

There were a couple of old-guest-breaking regressions.  I think we've 
fixed all of them but there could be more.  Are you using the latest 
kvm-userspace?

Regards,

Anthony Liguori

> virtio-net header not in first element
>
> This is because qemu expects a message with mergeable rx bufs (12
> bytes), but if I see things correctly the old kernel doesn't support
> that feature (sends 10 bytes). I put in some debug messages on IO
> reads/writes in the qemu virtio handler and got these:
>
> virtio IO read: 0 = 0x100bba3
> virtio IO write: 0x4 = 0x100bba3
> virtio: setting features 0x100bba3
>
> So I believe the feature masking is somewhat non-functional? Or did I
> read the masking code in qemu's virtio-net wrong?
>
> Basically for me, current KVM git broke old guest support, which is a
> clear regression and should not happen. Does anybody with more expertise
> in virtio want to take on this?
>
> Alex
>