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

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

[Avi Kivity]

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):
>
> 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?
>   

I think this was reported and fixed, possibly by c45a681.

-- 
error compiling committee.c: too many arguments to function

Re: virtio breakage with 2.6.25 guest kernel

[Mark McLoughlin]

On Tue, 2009-01-13 at 22:05 +0100, Alexander Graf wrote:

> On 13.01.2009, at 21:14, Anthony Liguori <anthony@codemonkey.ws> 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?

See:

  http://lists.gnu.org/archive/html/qemu-devel/2009-01/msg00574.html

> 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.

Nice idea, but no way of making the guest through feature detection
negotiation, I don't think.

Cheers,
Mark.

Re: virtio breakage with 2.6.25 guest kernel

[Rusty Russell]

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On Wednesday 14 January 2009 19:52:58 Mark McLoughlin wrote:
> On Tue, 2009-01-13 at 22:05 +0100, Alexander Graf wrote:
> 
> > On 13.01.2009, at 21:14, Anthony Liguori <anthony@codemonkey.ws> 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?
> 
> See:
> 
>   http://lists.gnu.org/archive/html/qemu-devel/2009-01/msg00574.html
> 
> > 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.
> 
> Nice idea, but no way of making the guest through feature detection
> negotiation, I don't think.

Just add a feature "VIRTIO_F_LIES_ABOUT_FEATURES"?  No guest should ever set this.

But I'm not sure it's worth the pain...
Rusty.

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