Page MenuHomeFreeBSD

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

Authored by royger on Jan 13 2022, 3:37 PM.
Tags
None
Referenced Files
Unknown Object (File)
Wed, Mar 27, 1:46 PM
Unknown Object (File)
Sun, Mar 10, 10:12 AM
Unknown Object (File)
Sun, Mar 10, 10:12 AM
Unknown Object (File)
Sun, Mar 10, 10:12 AM
Unknown Object (File)
Thu, Mar 7, 7:14 AM
Unknown Object (File)
Jan 11 2024, 1:47 PM
Unknown Object (File)
Jan 9 2024, 3:38 PM
Unknown Object (File)
Jan 3 2024, 5:13 PM

Details

Summary

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
system:

https://lists.xenproject.org/archives/html/xen-devel/2021-12/msg01581.html

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.

Diff Detail

Repository
rG FreeBSD src repository
Lint
Lint Not Applicable
Unit
Tests Not Applicable