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

[Michael S. Tsirkin]

On Mon, Feb 04, 2013 at 10:30:47AM +1100, Vadim Rozenfeld wrote:
> On Sun, 2013-02-03 at 15:09 -0600, Anthony Liguori wrote:
> > 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.
> > >
> > > So what do we do?  It isn't nice to break existing setups.
> > > Maybe mq can be disabled by default (together with Antony's
> > > patch) until fixed drivers will be more widely available?
> > 
> > I've got a patch that does exactly like this.  It's pretty ugly though
> > because of the relationship between virtio-pci and virtio-net.  I'm
> > going to try to see if I can find a cleaner way to do it on Monday.
> > 
> > I'm also contemplating just disabling mq by default.  Since a special
> > command line is needed to enable it anyway (on the backend), having to
> > specify mq=on for the device doesn't seem so bad.
> > 
> > But yeah, we don't want Windows guests to break with -M pc by default so
> > we should do something to work around it.
> > 
> > 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.
> > 
> > > 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
> 
> Why not just use revision id for that?

This will break well-behaved linux guests.

> It really can be very useful, at
> least for virtio Windows drivers.
> BTW, Yan already fixed this problem in the Windows driver code. So we
> can make a new build and make it public. But it probably will not be
> WHQL'ed in the nearest future.
>  
> > 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.
> > 
> > Regards,
> > 
> > Anthony Liguori
> > 
> > >
> > > How difficult it is to fix it in win driver?
> > >
> > > Thanks,
> > >
> > > /mjt
>