Re: Is: SKB_MAX_LEN bites again. Was: Re: bug disabling guest interface

Konrad Rzeszutek Wilk
Tue, 12 Mar 2013 14:49:41 -0000

On Tue, Mar 12, 2013 at 10:48:07AM -0400, Konrad Rzeszutek Wilk
wrote:
> On Fri, Mar 08, 2013 at 11:09:56PM +0100, Jacek Milewicz wrote:
> > > > > Is there any further information on a possible fix for
this?
> > Thoughts?
> > > > > Ideas? As it seems to be hitting multiple sites,
I''d like to get it
> > > > fixed asap...
> > > >
> > > > There are 3 ways I can see this fixed:
> > > > - update guests so all have same MAX_SKB_FRAGS (that
includes windows
> > > > drivers (windows drivers use 19 for MAX_SKB_FRAGS)) -add
some sort of
> > > > negotiation between host and guest
> > > > - change MAX_SKB_FRAGS to 19 to accommodate all guests
> > > >
> > > > Unfortunately first one requires changes to the guest and
most don''t
> > > > have that luxury. So the only way I see it could be fixed
without
> > > > breaking compatibility even more  is
> > >
> > > Ugh. The negotiations between host and guest is probably the best
> > choice.
> > > The issues you are going to hit are that you might need to redo
the skbs
> > to
> > > match what the frontend''s max is.
> > >
> > > Annie, Wei, Ian - were there some RFC patches floating around for
this?
> > >
> >
> > As much as I agree that negotiation is the best option in long term.
It
> > relies on people upgrading their kernels - and this is not the case
(only
> > people running kernels from before 2011-12-23 are affected by this bug
> > (I''m not sure about other platforms)).
> > And in a lot of cases (VPS providers) there is no easy way to upgrade
> > guest kernels.
>
> Sure. But the other option in the xen-netback might be a configurable
option
> to let net-back know that it is running with older guests which expect
> 18 size MAX_SKB_FRAGS. That way we have the negotiation part, the code to
> deal with backend MAX_SKB_FRAGS != frontend MAX_SKB_FRAGS and the value
that
> the system admin provides.
>

Re-sending as I think it never made it out.