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Re: [Xen-devel] Is: SKB_MAX_LEN bites again. Was: Re: bug disabling guest interface

  • To: xen-devel@xxxxxxxxxxxxx
  • From: Steven Haigh <netwiz@xxxxxxxxx>
  • Date: Sat, 09 Mar 2013 13:19:40 +1100
  • Delivery-date: Sat, 09 Mar 2013 02:20:24 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xen.org>

On 9/03/2013 9:09 AM, Jacek Milewicz wrote:
Is there any further information on a possible fix for this?
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
The issues you are going to hit are that you might need to redo the skbs
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.

For what its worth, I've increased the value of MAX_SKB_FRAGS to 19 on my Dom0 kernel and pushed out new packages... I'll see if I get any further reports...

Steven Haigh

Email: netwiz@xxxxxxxxx
Web: https://www.crc.id.au
Phone: (03) 9001 6090 - 0412 935 897
Fax: (03) 8338 0299

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