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xen/netfront: deal with mbuf data crossing a page boundary

Authored by royger on Jan 13 2022, 3:37 PM.
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There's been a report recently of mbufs with data that crosses a page
boundary. It seems those mbufs are generated by the iSCSI target

In order to handle those mbufs correctly on netfront use the bus_dma
interface and explicitly request that segments must not cross a page
boundary. No other requirements are necessary, so it's expected that
bus_dma won't need to bounce the data and hence it shouldn't
introduce a too big performance penalty.

Using bus_dma requires some changes to netfront, mainly in order to
accommodate for the fact that now ring slots no longer have a 1:1
match with mbufs, as a single mbuf can use two ring slots if the data
buffer crosses a page boundary. Store the first packet of the mbuf
chain in every ring slot that's used, and use a mbuf tag in order to
store the bus_dma related structures and a refcount to keep track of
the pending slots before the mbuf chain can be freed.

Reported by: G.R.
Tested by: G.R.

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rG FreeBSD src repository
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