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

[Alexander Graf <agraf@suse.de>]

Mark McLoughlin wrote:
> Hi Alex,
>
> (Dropping kvm-devel@sf.net - it's kvm@vger.kernel.org now anyway)
>   

Thunderbird keeps selecting that one and I'm sometimes too confused to
see it's actually the wrong one.

> On Tue, 2009-01-13 at 14:33 +0100, Alexander Graf wrote:
>
>   
>> 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?
>>     
>
> Yeah, I noticed this one last week.
>
> The issue here is that 2.6.25 virtio erroneously says it supports all
> features supported by the host - i.e. it's a guest kernel bug.
>
> With recent qemu, that causes us to enable mergeable receive buffers and
> the guest barfs. See:
>
>         mask = ioread32(vp_dev->ioaddr + VIRTIO_PCI_HOST_FEATURES);
>         if (mask & (1 << bit)) {
>                 mask |= (1 << bit);
>                 iowrite32(mask, vp_dev->ioaddr + VIRTIO_PCI_GUEST_FEATURES);
>
> This got fixed in 2.6.26 by:
>
>   http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=c45a6816
>
> I did consider forwarding this to stable@vger.kernel.org, but apparently
> the 2.6.25.x series is finished now anyway.
>   

Ok, that's good to know. I don't think anyone officially supported
virtio in 2.6.25 days, but I'll check internally if we need to backport
anything.

Thanks a lot

Alex