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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Mar 08, 2013 at 08:36:37PM +0100, Jacek Milewicz wrote:
> Hi All
> 
> > 
> > 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?

> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 821c7f4..82de0f5 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -143,8 +143,8 @@ struct sk_buff;
>   * Since GRO uses frags we allocate at least 16 regardless of page
>   * size.
>   */
> -#if (65536/PAGE_SIZE + 1) < 16
> -#define MAX_SKB_FRAGS 16UL
> +#if (65536/PAGE_SIZE + 1) < 19
> +#define MAX_SKB_FRAGS 19UL
>  #else
>  #define MAX_SKB_FRAGS (65536/PAGE_SIZE + 1)
>  #endif
> 
> 
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> 

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