[ofa-general] Re: [PATCH 0/3] ib/ipoib: Enable IPoIB-UD 4K MTU support

Shirley Ma xma at us.ibm.com
Fri Feb 1 16:36:50 PST 2008





Hello, Roland,

Thanks for your quick review.

> Not sure I understand this.  Is there any possible configuration of
> any architecture where Linux runs where PAGE_SIZE < 4096?

Technially it's a problem, pratically it's not since there is no
architecture i can think of has PAGE_SIZE < 4096.

>  > This patch allows IPoIB-UD MTU up to 4092 (4K - IPOIB_ENCAP_LEN) when
>  > HCA can support 4K MTU. In this patch, APIs for S/G buffer allocation
in
>  > IPoIB-CM mode has been made generic so IPoIB-UD and IPoIB-CM can share
>  > the S/G code.
>
> This approach seems overly complex to me, since it ends up going
> through all the CM buffer fragment bookkeeping for the simple UD path.

No, it's not complex, only one buffer is allocated if the page_size is
bigger enough and if it's 2K MTU.
> So how about the following?  When using a UD MTU of 4096 with a page
> size of 4096, allocate an skb of size 44 for the GRH and ethertype,
> and then allocate a single page for the fragment list.  This means
> that the IP packet will start nicely 16-byte aligned for free, and all
> the bookkeeping is very simple.

It has 44 bytes head with another 4K page size without if condition check
of mtu size and page size. Please look at the patches for detail.

thanks
Shirley
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