Re: [Qemu-devel] Headsup: windows virtio networking does not work on current git

[Rusty Russell]

Anthony Liguori <address@hidden> writes:
> Michael Tokarev <address@hidden> writes:
>
>> 03.02.2013 17:23, Yan Vugenfirer wrote:
>>
>>>> If it helps, mq changes the config size from 8 to 16 bytes.  If the
>>>> driver was making an assumption about an 8-byte config size, that's
>>>> likely what the problem is.
>>>>
>>> That's exactly the problem.
...
> N.B. this is a pretty nasty bug in the guest driver.  Any new virtio-net
> feature is going to trigger it (not just multiqueue).  So while pc-1.3
> will work forever with this guest image, it's pretty much guaranteed to
> break at some point in the future.

Wow, yes.  I never expected such a bug.  I probably don't even want to
know how this happened, right?

If I could find a way, I'd like to create some code as an appendix to
the virtio spec which would torture test each driver and/or device by
configuring it in strange ways.  But that's pure speculation at this
point.

>> It's easy to turn off mq by default and turn it on when
>> needed...
>>
>> The problem now is that it isn't obvious why your existing
>> setup breaks when you upgrade.  While when you especially
>> play with mq, you may look at options available around it...
>
> If we ever to get virtio2, a really handy feature to have would be a
> driver identification string.  Even if was only informative (and not
> authoritative, we've had a lot of issues like this and being able to
> make work arounds conditional on the driver identification string would
> be nice.

In practice, we'd want to formalize it into a string and a version, and
hope the version gets bumped appropriately.  Because it'll actually be
used for bug detection.

But AFAICT that would be useless in this case.  We really want to know about
the guest before we even start it.

Cheers,
Rusty.