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

[Alexander Graf]

On 13.01.2009, at 21:14, Anthony Liguori <address@hidden> wrote:

> 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?

This one is definitely due to the broken guest kernel. I applied the  
patch mark mentioned to ours and things started working.

So the only way I can think of to 'fix' it is by detecting broken  
guests. We could supply a host mask of 0xffffffff and see if tge guest  
feature mask is tge same. If so, feature masking is probably broken.

Alex

> 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