Re: [Xen-devel] 3.0.4 (and earlier): skbuff_ctor() use of xen_create_contiguous_region()

[Keir Fraser <keir@xxxxxxxxxxxxx>]

On 9/3/07 12:37, "David Edmondson" <dme@xxxxxxx> wrote:

> So that a multi-page packet is physically contiguous when passed to
> the NIC for transmit?
> 
> If it's not physically contiguous and the NIC hardware can't do
> scatter gather then the driver has to re-allocate and copy.

Yes, it was originally intended for drivers that would do dma_map_single()
on the whole multi-page network buffer (for receive and/or transmit
buffers). Of course we still potentially have this issue and will now end up
falling back to swiotlb at the time of the dma_map_single() call. It's only
(usually) an issue for jumbo-frame systems since kmalloc() will not usually
return page-straddling allocations for sub-page requests. Also the custom
allocator did not help the transmit-from-domU case since jumbo packets are
always fragged across netfront/netback, and then many drivers will handle
fragged packets efficiently. But it could have helped the receive-to-domU
case by avoiding an extra copy, as most drivers want to allocate a single
contiguous buffer for jumbo frames.

 -- Keir


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